Question

Sarah is twice as old as John. Six years ago, Sarah was 4 times as old as John was then. How old is John now?

Answer 1

$J- John's\ age\ now\\S-Sarah's\ age\ now\\\\S=2J\ \ \ and\ \ \ S-6=4(J-6)\ \ \ \Rightarrow\ \ \ 2J-6=4J-24\\\\2J-4J=-24+6\\\\-2J=-18\ /:(-2)\\\\J=9\\\\Ans.\ Now\ John\ is\ 9.$

Answer 2

S = 2J
S - 6 = 4(J - 6)
S - 6 = 4J - 24
2J - 6 = 4J - 24
-2J = -18
-J = -9
J = 9

18 = 2(9)
12 = 4(9 - 6)
12 = 12!