Hi,


BEFORE
A [______x______]
B [______x______]

AFTER
A [__x - 120__]
B [____________x + 120____]

A = x - 120
B = 4A = x + 120

4 * (x - 120) = x + 120
4x - 480 = x + 120
4x - x = 480 + 120
3x = 600
x = 200

Thus A + B = 2x = 2 * 200 = 400

I'm not from Singapore. When I did study ages ago (I'm old now, got my son in P3 this year), my teacher told me different approach on solving this. I don't know in Singapore whether it is allowed to use other approach as long as the result is correct. Anyway, I would like to share with you the other approach.

A = x - 120
B = 4A = x + 120

x + 120 = 4A
x - 120 = A
----------------- - ===> Substract to remove x, thus only 1 variable left
240 = 3A
A = 80
B = 4A = 4 * 80 = 320
A + B = 80 + 320 = 400

I hope my explanation helps