Part A
Initially
the sub is divided into 4 equal parts
so
each part is 1/4
Now
The sub must be divided into 5 equal parts
each part must be 1/5
that means
Each person will get less sub
because
1/5 < 1/4
Part B
Divide each piece of 1/4 by 5
so
(1/4) : 5=1/20
Each person will get 4*(1/20)=4/20
simplify
4/20=1/5