Suzy can paint a house in 4 hours. Peter can paint the same house in 6 hours. How long will it take to paint the house if they painted it together?
I solve these by the following:
1 house/4 hours = 0.25 house/hr.
1 house/6 hours = 0.157 house/hr.
If they paint together they will have those combined or
0.25 house/hr + 0.167 house/hr = 0.417 house/hour.
or 1/0.417 hours/house = 2.4 hours.