# Algebra 1 – Find the equation of a line

Q: If the slope of the line through (-3,6) and (1,y) is 1/2, what is the value of y?

Explanation: Slope is computed by the formula (y2 – y1) / (x2 – x1) … the change in y divided by the change in x.

So,

(y – 6) / (1 – -3) = 1 / 2

(y – 6) / 4 = 1 / 2

Multiply both sides by 4 and you have:

y – 6 = 2

y = 8

BUT, if you’re taking a test and having trouble remembering the method above, try this instead: Plot the point (-3,6) on a graph and move from that point with a slope of 1/2. Move up 1, right 2, up1, right 2… and repeat this pattern until your x-value is 1. Then, read the y-value at the same point. This is not my favorite way, but it works!

And one more thing!  If this were a multiple choice question, you could use my TI-84 Plus program ‘DISTANCE’ (which finds distance, midpoint, and slope for any two ordered pairs).  Substitute one of the answer choices for x, and see if the program reports that slope is 1/2.

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# SAT – Counting and Ordering

Q: In how many ways can the letters of the word ‘FIVE’ be arranged?

Explanation 1:  SInce there are 4 different letters, do 4!  (that’s ‘4 factorial’).   4! = 4 x 3x 2x 1 = 24.  Your calculator might have a factorial (!) function.  On the TI-84 Plus, find it in MATH – PRB.

Explanation 2:  SInce you are choosing 4 items from a total of 4 items, and the order is important, you can do 4 nPr 4, or use the formula for nPr.

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# SAT – number properties

Q: How many positive integers less than 500 are multiples of 3, 7, and 11?

Explanation:  3, 7, and 11 do not have any common factors except 1.  So, their least common multiple is 3 x 7 x 11 = 231.  The next two common multiples are 231 x 2 = 462 and 231 x 3 = 693.  693 is greater than 500.  There are only two that are less than 500.  They are 231 and 462.

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# SAT – Counting and Ordering

Q:  When 5 people met for dinner, each person shook hands with each other person once.  How many handshakes happened?

Explanation 1: Two people = a handshake.  This question is really ‘How many ways are there to choose 2 people from a total of 5, if the order is not important?’ It’s a combination question. Find the nCr function on your calculator. Type in 5 nCr 2. It’s 10. (There is a longer way to do it by hand, using the formula for nCr.)

Explanation 2: Person A shook hands with 4 people.  That’s  4 handshakes so far. Person B shook hands with 3 other people (his shake with A has already been counted). Person C shook hands with 2 other people.  Person D shook hands with 1 other person (E). Add: 4 + 3 + 2 + 1 = 10.

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# Algebra 1 – Find the equation of a line

Q:  Find the equation of the line that has the following intercepts:

x-intercept: 5,   y-intercept: -3

Answer:  y = 3/5 x – 3

Explanation:  The x-intercept is the point (5, 0).  The y-intercept is the point (0, -3).  Calculate the slope of the line passing through these points:

m= (0- -3)/(5 – 0) = 3/5

Using slope intercept form (y = mx + b), the equation is y = 3/5 x + b.  We just need to find b.  Or do we?  We could plug in an ordered pair and solve for b, but it’s not necessary.  We already have b, because in slope-intercept form, b always stands for the y-intercept.  So, b = -3.

The equation is: y = 3/5 x – 3

My TI-84 Plus program ‘LINES’ will find this equation for you, given the two intercepts.

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# SAT – Volume

Q: A cylinder has a height of 20 cm and a diameter of 30 cm. To the nearest cubic centimeter, what is the volume of the cylinder?

Explanation:  Use the formula for the volume of a cylinder, with r = 15 and h = 20.  Round to the nearest whole number.
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# SAT – Percent change

Q: If the length of a rectangle is increased by 30%, while the width is decreased by 10%, by how much will the area increase? Give the answer as a percent.

Explanation:  Choose the length and width of the original rectangle to be 10.  Then, the length and width of the new rectangle are 13 and 9 (by increasing 30% and decreasing 10%).  The original area is 10 x 10 = 100.  The new area is 13 x 9 = 117.  The percent increase from 100 to 117 is (117-100)/100 = 17/100 = 17%.
Note that if this were an SAT problem with answer choices, you should immediately eliminate any ‘too easy’ answers, such as those related to the numbers 30 and 10 too directly.  Eliminate 20% and 40%.
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# Geometry – Volume

Q: What is the maximum number of boxes with dimensions 4”x5”x8” that can be packed into one large box with dimensions 20”x20”x24”?

Explanation: First, decide that a whole number of small boxes will entirely fill the large box.  To do this, look at the dimensions and match them.  4″ fits into 20″ five times.  5″ fits into 20″ four times.  8″ fits into 24″ three times.  That means, you’ll get five rows of small boxes, four boxes per row, and three layers deep.  So, the quickest way to get the answer is to multiply 5x4x3 = 60.

BUT if you don’t notice that, just do (large volume)/(small volume).

Large volume = 20x20x24 = 9600

Small volume = 4x5x8 = 160

(large volume)/(small volume) = 9600/160 = 60

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# SAT – remainders

If summer begins on a Tuesday and ends 100 days later, on which day of the week does summer end?

Explanation:  A week has 7 days.  Dividing 100 by 7, you get 14 remainder 2.  That’s 14 weeks and 2 days.  Beginning on a Tuesday, 14 weeks later would land on a Tuesday too.  Two more days brings you to Thursday.

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# Algebra 1, 2 – systems of equations

Two cubes and one pyramid weigh 9 lb. Seven cubes and two pyramids weigh 24 lb. All cubes weigh the same, and all pyramids weigh the same. What is the weight of one pyramid?

Explanation:  Let x be the weight of a cube.  Let y be the weight of a pyramid.

The first sentence becomes: 2x + y = 9.

The second sentence is: 7x + 2y = 24.

Solve this as a system of equations by any method you choose… substitution, elimination, Cramer’s Rule, etc.  Or use my TI-84 Plus program ‘CRAMER’.

x = 2, y = 5

‘y’ is the weight of one pyramid, so the answer is 5.