John and David can finish a job together in 1 hours. If John can do the job by himself in 2 hours, what percent of the job does David do? Choices:
A) 40 %
B) 45 %
C) 50 %
D) 55 %
E) 60 %

Rationale: you can also plug in numbers . for example , bob and alice work at a donut factory and make 10 donuts which is the job ( i picked this as a smart number ) . john on his own works 10 / 2 = 5 donuts per hour . john and david work 10 / 1 = 10 donuts per hour so david works 5 donuts / hour to find out the percentage , david works 5 donuts / hr x 1 hours = 5 donuts per hour . therefore 5 donuts / 10 donuts = 1 / 2 = 50 % answer : c
Answer: C