Answer
The fractions in the pie chart must add up to $1$ because a fraction is given for every "piece" in the chart, with no gaps.
Work Step by Step
We know that the fractions in the pie chart must add up to $1$ because a fraction is given for every "piece" in the chart, with no gaps. We know that the sum of the parts must equal the "whole" and in the case of fractions, the whole means $1$. For example, if a pie is split in two parts, we would have $1/2+1/2=1$ and similarly for other combinations of fractions.
