It usually takes Mary 6 hours to do the cleaning that Jane can do in 5 hours and Ellen can do in 4 hours. If Jane and Ellen agree to help Mary so that they can all go on a picnic, how long will it take them to do the
cleaning?

It will take them about 5 hours to do the cleaning.

Step-by-step explanation:

Since all three of them are going to do it together, you must find the average amount of time between the three of them. To do this you add all of their cleaning times, 6, 5 and 4, and them divide by the amount of people there. 6+5+4= 15, 15/3=5.

v: l•w•h÷2.

v: 20•6•8 ÷2

v: 960÷2

v: 480