Now,
G - Ginny's age.
P=G+8, Percy's age.
In 10 years,
G + 10 - Ginny's age in 10 years.
P+10 - Percy's age in 10 years.
In ten years time Percy (P+10) will be three times Ginnys present age (3G).
P+10=3G
Now we have 2 equations, and we can write a system of equations.
(1) P=G+8
(2) P+10=3G -------> G=(P+10)/3, we need to substitute it into equation 1.
( P=(P+10)/3+8) /*3
3P=P +10+24
2P = 34
P = 17
Percy is 17 years old now.
Check,
P=17, G=17-8=9
P+10=3G , 17+10=3*9, 27 = 27 True