Todd ate half of the apple. If his sis ate 1/8 of the apple that was left (1/2), there would be 7/16 of the apple left over. (try cutting the apples into 16 different slices. Todd eats half, so 8 is left. Then, his sister eats 1/8 of 8 slices. That means she ate 1 slice. There are 7 slices left. 7/16)
