SATMathematics Practice Test 1

Substituting 6 for x and 24 for y in $\mathit{y}=\mathit{k}\mathit{x}$ gives 24 = (k)(6), which gives $\mathit{k}=4$. Hence, $\mathit{y}=4\mathit{x}$.
Therefore, when x = 5, the value of y is $\left(4\right)\left(5\right)=20$.
None of the other choices for y is correct because y is a function of x, and so there is only one y-value for a given x-value.
Choices A, B, and D are incorrect.
Choice A is the result of using 6 for y and 5 for x when solving for k.
Choice B results from using a value of 3 for k when solving for y.
Choice D results from using $\mathit{y}=\mathit{k}+\mathit{x}$ instead of $\mathit{y}=\mathit{k}\mathit{x}$.