Age problem algebra

Question

carlo is four years older then his sister. Seven years ago he was three times older then his sister. How old is his sister now?

Answer 1

c = 4+s, c-7 = 3(s-7) = 3s -21, c = 3s -14 = 4+s, 2s = 18, s= 9 yrs old = sister's age

Carlos is now 13

7 years ago they were 2 and 6. 6 = 3x2