Ben brought two pizzas to a party. he says that sinceramente 1/4 of each pizza is left,the sale a mountain of each pizza is left. what is his error?

The correct question in the attached figure

Let 
x------------------------ > remaining amount of pizza n 1-------> 1/4
y------------------------ > remaining amount of pizza n 2-------> 1/4

we have that
x=y
therefore
the same amount of each pizza is left
There is no error.

To find a residual you must take the predicted value and subtract it from the measured value.