我細佬既數學題....好易,,,(5分)

Let David's current age be A
and Sam's current age be B

A = B + 9 ----------------------------> (1)

(A+5)/2 = B + 5
A + 5 = (B+5)*2
A = 2B + 10 - 5
A = 2B + 5 ----------------------------> (2)

(1) = (2)
B + 9 = 2B + 5
9 - 5 = 2B - B
B = 4 ----------------------------> (3)

Put (3) into (1)
A = B + 9 (B=4)
A = 4 + 9
A = 13

David is 13 years old, and Sam is 4 years old now. In 5 years, David will be 18 years old and Sam will be 9, which is half of David's age then.

Let's David's age is x years old, Sam's age is (x-9) years old,
After 5 years,
David's age will be (x+5), Sam's age will be (x-9+5):

(x+5)=2(x-9+5)
x+5=2(x-4)
x+5=2x-8
5+8=2x-x
13=x
x=13

So David's age is 13 now.

Let david is y years old now.

(y-9)+5=(y+5)/2
y-4=(y+5)/2
2y-8=y+5
y=13

So,David's age is 13 now.