GMAT Problem Solving (PS) | Re: John takes 15 hours to complete a certain job, while Bill takes only 6

Its Option B here.

John can complete the work in 15 hours so the amount of work completed in hour= 1/15

Bill can complete the work in 6 hours, so the amount of work complete in one hour= 1/6

Add above two,

1/15+ 1/6 = 7/30.

Now Test the answer choices,

Option 1: 7/30+ 1/ X = 2/5 => X= 6 hours.

Option 2: 7/30 + 1/X= 4/11 (x=Around 7. 5 ish...)

Option 3: 7/30 + 1/x = 3/10 (X is 15)

OPTION 4 and 5 will be definitely greater than 15.