For the christmas dance ,the dance committee needs three hours of music. each song is an average of 3.5 minutes . how many song do they need

An hour is 60 minutes

there are three hours so 
60*3=180

how many times will 3.5 go into 180 so 
180/3.5=51.43 or rounded up you get 52 songs (if they're good, they'll play 52 average songs and one half legnth songs)
so they will need 52 songs

There are 60 minutes in an hour, 3 hours to fill up, therefore the problem should be 180/3.5.
They need about 52 songs.
Hope this helped!

Cdoes not have two variables