Propagation of Errors - Upper Bound

Question

Let $p, q$ and $r$ be $3$ quantities in a calculation. Assuming that values are rounded 3DP. Calculate the upper bound on the absolute error when calculation $$p+q+r$$

I don’t even know where to start! Someone help please

Answer 1

For each variable if it is rounded to 3 decimal places then the error will be between -0.0005 and +0.0005 (inclusive). Since the three will add the sum of the errors will be between -0.0015 and +0.0015 (inclusive).

Thus the absolute error will be at most 0.0015.