Peter is three years older than his sister paula. If their combined age is 21 find their actual ages.

Question

x+3=21
X+3-3=21-3
X=18

Answer=18 and 3

Answer 1

No,

x + x + 3 = 21

2x = 18

x = 9

Paula is 9 years old.

Answer 2

Your answer does not satisfy then condition that Peter is 3 years older than Paula.
Shouldn't your equation be
Paula --- x
Peter --- x+3

x + x+3 = 21
2x = 18
x = 9

So Paula is 9 and Peter is 12

Answer 3

(21-3)/2
18/2=9
9paula and Peter 9+3=12