Question

if a boy is 4 years less than twice his sisters age and the sum of the age is 11 how old is he

Answer 1

go through each of the possibilities of getting to 11. 1+10 2+9 3+8 4+7 and so on. think of what you are looking for, twice the girls age will be four more than the boys age.

Answer 2

or, using algebra
b+s=11
b=2s-4

2s-4+s=11
3s=15
s=5
so, b=6

check:
6=2*5-4