Q: The variables a, x, and b are positive integers. If ax = 7, xb = 28, and ab = 4, what is the value of a?
a, x, and b are all positive integers, so the only possibilities for the variables are factors of 7, 28, and 4.
Since 7 is prime, a and x must be 7 and 1, but in which order? If x = 1, then b = 28, and then we can’t find an integer value for a from the last equation. So, x must instead be 7.
It follows that a = 1 and b = 4.
The answer is 1.