The residuals of any data set should get added to zero. If there are three residuals (say A, B, and C) in any data set, then it could be written as A + B + C = 0.
Given are the two residuals -10 and -20 out of three point in a particular data set.
Let's assume A = -10 and B = -20, so that we could find third value C of the given data set.
We know A + B + C = 0
(-10) + (-20) + C = 0
(-30) + C = 0
C = 30
It means that third residual must be 30.
Hence, the final answer is 30.