Uncertainties Definition and 152 Threads

A quick question about uncertainties and significant figures: Say, we have some numbers with a particular uncertainty 0.1 of each of them. What happens if the sum of these numbers has a greater amount of significant figures than each of the numbers alone? For instance, 1.01 + 9.99 = 11.0...

I am trying to find F, i have L = 60cm and U = 31cm, how would i calculate f (focal length)? and also how would i find the uncertainties? thank you very much