Question

One student can paint a wall in 20 minutes. Another student can paint the same wall in 30 minutes. Working together, how long will it take for them to paint the wall?
10 minutes

12 minutes

15 minutes

25 minutes

Answer 1

1) in one minute for first  wiil be 1/20

the other 1/30, it means that  A paint in one minute  1/20 and B 1/30 then   A+B in one minute 1/20 + 1/30 = 3+2/60 = 5 / 60,  5/60 = 1/12

if in one min they make 1/12 then  in 12  min they  paint all the wall