Well, just bear in mind that Work = Rate x Time and you're good to go.
Sample problem:
If Ann can clean the house in 1 hr and May can clean the house in 2hrs., how long will it take them if Ann and May cleans the house together, but May leaves Ann to clean alone after 30 min.?
Rate = Work / Time
Ann's rate is 1/1 = 1; May's, 1/2.
W = rt
1 = (1 + 1/2)(1/2) + (x)
1 = 3/4 + x
x = 1/4
So, Ann works for 1/4 hr., 15 min., after May leaves.
It took them 30min. + 15min. = 45 min.
