How much money does the sister have?

Ed has $e$ dollars, and his sister has $s$ dollars. We can set up a system of equations: $e+s=100$ $e=s+90$

If we substitute $e$ for $s+90$ in the first equation, we get: $2s+90=100$

which can be simplified to:

$\boxed{s=5}$

First, you minus 90 from 100 to get 10 which you should divide by two to get 5 which will be the sisters amount.

However, Ed doesn’t seem to be a really good brother because how can he have $90 more than his sister? He must be like really unfair and greedy and he probably stole the money away from his sister so perhaps he should get a better education but I bet that he got expelled quite a lot of times judging by his behaviour. Anyways, his mum should also give so,e more effort into getting rid of her child’s bad habits but it seems like Ed comes from a family of thieving so the mother obviously doesn’t care. Also, Ed really needs to apologise to his sister and make sure that he never ever commit another crime.