algebra
posted by Kim on .
John takes 3 hours longer than Andrew to peel 400 pounds of apples. Together they take 8 hours. How long will it take John to peel 400 pounds by himself

This should give you the information you need.
If it takes one person 5 hours to paint a room and another person 3 hours, how long will it take to paint the room working together?
Method 1:
1A can paint a room in 5 hours.
2B can paint a room in 3 hours.
3A's rate of painting is 1 room per A hours (5 hours) or 1/A (1/5) room/hour.
4B's rate of painting is 1 room per B hours (3 hours) or 1/B (1/3) room/hour.
5Their combined rate of painting is therefore 1/A + 1/B = (A+B)/AB = (1/5 + 1/3) = (8/15) rooms /hour.
6Therefore, the time required for both of them to paint the 1 room working together is 1 room/(A+B)/AB rooms/hour = AB/(A+B) = 5(3)/(5+3) = 15/8 hours = 1 hour52.5 minutes.
Note  Generally speaking (if the derivation is not specifically required), if it takes one person A units of time and another person B units of time to complete a specific task working alone, the time it takes them both to complete the task working together is T = AB/(A + B), where AB/(A + B) is one half the harmonic mean of the individual times, A and B.
You might like to derive the equivalant expression involving 3 people working alone and together which results in T = ABC/(AB + AC + BC).