Question 427705
If M + B + K can do a task in 1hr 20min then they take 1-1/3 hours to do one task, or they can do 3/4 task per hour.  Let's say M can do 1/M amount of the task per 1 hour, then B=1/2M or M/2.  Since M takes 2 hrs longer than K, K=M-2.

To set up our equation, we have:

1/M + 1/M/2 + 1/M-2= 3/4
1/M + 2/M + 1/M-2= 3/4
3/M + 1/M-2= 3/4

Multiplying through by 4(M)(M-2), we get:
12M-24+4M = 3M^2-6M
0 =3M^2-22M+24
0 = (3M-4)(M-6)
M =4/3 or 6