Question

Mr. Anders was three times as old as Kate 5 years ago. Their total age now is 42 years. How old is Kate now?

Answer 1

We can write this as two equations. Call Mr. Anders' age A and Kate's age K:
$A-5=3(K-5)$
$A+K=42$

Then solve for A in the second equation:
$A=42-K$

Substitute this into the first equation:
$(42-K)-5=3(K-5)$
$-K+37=3K-15$
$52=4K$
$K=13$