Question

A student measures two numbers x and y as x=10+-1 and y=20+-1 What is her best estimate for their product q=xy?

Answer 1

again the relative errors (faults) in a product need to be added:
the relative errors are
for x = r.f. = 1/10 = 0.1
for y = rf. = 1/20 = 0.05
added rf = 0.15
q = x.y = 200 with rf 0.15
so absolute error is 200*0.15 = +-30
so q = 200 +-30