Question

After John worked at a job for 10 years, his salary doubled. If he started at $x, his salary after 10 years is _____.

$x
$x + 2
$x - 2
$2x

Answer 1

His salary doubled (x2), so you can eliminate the 2nd & 3rd ones, because they are + & -. You can eliminate the first one because that would represent his current salary. So, that leaves the last one $2x.

Answer 2

What does it mean that his salary doubled?

this means that for example, if it was 1000 dollars, it would be 2000 dollars now, and it if was 2000 dollars, it would be 4000 dollars now.

This can be written down as $2x where x is the initial salary. (so if x=1000, the new salary is 2x=2*1000=2000