The correct answer of this question is 10.
let the present age of mother= M
....................... C....= C
by first condition
M+10=4(C-10)
M-4C=-30.........eq(1)
Now the issue is in second condition...Here the mother age is compared with the present age of C, not the future age...so equation will be some how like
M+10=2C.....and not M+10=2(C+10)
so the second equation is
M-2C=-10.....eq(2)
solving simultaneously we get C=10....