7.
A – B = 25
(1) B = A – 25

A + 12 = 2(B – 12)
A + 12 = 2B – 24
2B = A + 36
B = 1/2(A + 36)

Substituting (1) into (2),
A – 25 = 1/2(A + 36)
2A – 50 = A + 36
A = 50 + 36 = 86 #

6.
Let A be the total number of balls in basket A at first.
Let B be the total number of balls in basket B at first.

A = B + 18
So (1) B = A – 18

3(A – 64) = B + 64
3A – 192 = B + 64
=> (2) B =3A – 256

Substituting (1) into (2),
A – 18 = 3A – 256
2A = 238
A = 119 #

5.

A = 425
B = 189

Let X be the number of marbles transferred.

A – X = B + X
A – B = 2X
=> X = (A – B)/2 = (425 – 189)/2 = 118 #

4.

A = R

5(A – 36) = R + 36
5A – 180 = R + 36
5A – R = 216

But since A=R,
5A -A = 216
4A = 216
A = 54

So both Andy and Rita has 54 stamps at first #