2015 CEMC Gauss (Grade 8) Problems/Problem 5

Problem

If $x + 3 = 10$ , what is the value of $5x + 15$ ?

$\textbf{(A)}\ 110 \qquad\textbf{(B)}\ 35 \qquad\textbf{(C)}\ 80 \qquad\textbf{(D)}\ 27 \qquad\textbf{(E)}\ 50$

We can solve for $x$ in the equation by subtracting $3$ from both sides, and then plug x into $5x + 15$ :

$x + 3 - 3 = 10 - 3$

$x = 7$

$5x + 15 = 5 \times 7 + 15 = 35 + 15 = \boxed {\textbf {(E) } 50}$

~anabel.disher

We can notice that $5x + 15 = 5x + 5 \times 3 = 5(x + 3)$ . Thus, we can simply multiply both sides of our equation to $5$ to get the answer:

$5(x + 3) = 10 \times 5$

$5x + 15 = \boxed {\textbf {(E) } 50}$

~anabel.disher