<PRE><B>Question 392388</B>
peter=P
david=D

first formula is P=12+D (because peter is 12 years older thn David)
the second formula is 4P+2D=66 (because four times Peter’s age plus two times David’s age is 66)

and then we substitute the first formula to the second formula
4P + 2D = 66
4(12+D) + 2D = 66
48 + 4D + 2D = 66
48 + 6D = 66
48 - 48 + 6D = 66 - 48
6D=18
6D : 6 = 18:6
D= 3

so David is 3 years old

P = 12 + D
P = 12 + 3
P = 15

so peter is 15 years old</PRE>