To add on to Wiggin42's answer (which is correct), I would like to take the time to explain the reason why this TA got this answer. In addition to the the given standard form formula:
There are also 3 rules that should be followed accordingly:
1. A shouldn't be negative.
2. A and B shouldn't both be zero.
3. And, A, B, and C should be integers.
#1 explains why the B integer is negative rather than the A integer. A can never be negative.
And, #3 explains the reason why the TA multiplied both sides of the equation by 20, they were not whole numbers. The original equation was written in decimal form. The TA found a common multiple that would eliminate the decimals (which was 20), and give us an integer. And, an integer is defined as a whole number.
Examples of an integer would be -2, -1, 0, 1, 2, etc.
Standard form is given by the formula:
Ax + By = C
where A, B, C are constant integers.
Multiply by 20 to get:
5x = 2 + 4y
Rearrange to get:
5x - 4y = 2
Therefore, A = 5, B = -4, C = 2