let x be the amount of cash
then 3/4x = A
1/2 (x - 3/4x) = B
A = 10 + B
using substitution 3/4x = 10 + B then B = 3/4x - 10
so 1/2(x-3/4x) = 3/4x - 10
1/2(1/4x) = 3/4x - 10
1/8x = 3/4x - 10
-5/8x = -10
x = 16
so total amount was $16 and A was 3/4 of that so 3/4(16) = 12