Abaker has 2/4 a cake on display another baker has 4/6 what will you get when you multiply

On average, how many more hits will he get when he bats 100 times at home during the day as opposed to 100 times away at night?

Which simplifys to a+ b? a-(1-b)+1 -(1-a)-b+1 (a--1) -1)+1

[10 points, algebra 2-simplifying complex fractions]according to my answer key the restrictions are -1 and 4. but why not positive 1?