posted by Kgk
Alma can grade a batch of placement exams in 3hr and Kevin can grade a batch in 4hr. if they work together to grade a batch of exams, what percentage of the exams will have been graded by Alma?
Assuming they both work the same time, Alma works 4/3 as fast as Kevin, so she does 4/7 of the work, or 57%
Or, you can find how long it takes to do the whole job together:
1/x = 1/4 + 1/3
x = 12/7
So, in 12/7 hours, Alma does 1/3 * 12/7 = 4/7 of the work.