The best way to approach this type of problem is to turn it into an actual scenario. Pretend that Sal's job requires him to mow 10 lawns. Since he completes this task in 5 hours, Sal must mow 2 lawns per hour. When Sal and Pat work together, they complete the tasks (mowing 10 lawns) in 3 hours. We already know that Sal mows lawns at a rate of 2 per hour. Since the pair worked for three hours, that would allow Sal to mow 6 lawns. However, since we already assumed that the job entails mowing 10 lawns, Pat must have mowed 4 lawns in that 3 hour time span. Thus, Pat does 40% of the job. (4/10 = 40%).