They give you both distributions for the torque and cap strength. You are concerned that T<C. Thus a new distribution must be created for the DIFFERENCE Y=C-T. You wish to know P(Y < 0). I'll leave you to fill in details. As for why you can assume independence; it's highly unlikely this company manufactures both the machine that applies the cap AND the cap; thus it's fair to assume that the strength of the caps are manufactured independent of knowledge of that specific machine. If that wasn't true (say the company buys caps from a distributor that has knowledge about the machines they use), it's unlikely the two r.v.'s would be independent.