let the acceleration of block A be a1 and block B a2 and the tension in string attached to A be T.

this implies the tension in string attached block B as 2T.

by using virtual work principal  2Tx2=Tx1

2a2=a1

5g-2T=5a2

T=2a1

5g=2T+5a2

5g=13a2

a2=5g/13

a1=10g/13