F.2 Maths Questions

Let P be the age of Peter now.
therefore the age of sister is P-7 now.
9 years ago, Peter's age is P-9, and his sister's age is P-7-9 = P-16
He was twice as old as his sister 9 years ago, so
P-9 = 2 * (P-16) = 2P-32
P = 23

Therefore Peter's age is 23 years old now.

ihaveans ( 碩士級 3 級 )
Thank you so much!