Set up an expression.
Each day he ate 6 more bananas, so
x will be the number of bananas he ate on the first day.
x + (x+6) + (x+6+6) + (x+6+6+6) + (x+6+6+6+6) = 100
5x +60 = 100
5x = 40
x = 8
Now on the 5th day, May 5, he ate (x+6+6+6+6), which is (8+6+6+6+6+6), which equals 38.
On May 5, he ate 38 bananas.