If they work simultaneously, how long will it be until the whole board is covered?

Question

bart covers the whole board in 50 mins whereas nelson takes 80 mins to erase the board.

Answer 1

1/50 = portion of board erased by Bart in 1 minute

 

1/80 = portion of board erased by Nelson in 1 minute

 

Let x = number of minutes that it takes both of them to erase the board if they work together 

 

Then, 1/50 + 1/80 = 1/x

 

Multiply by 400x:  8x + 5x = 400

 

                                 13x = 400      x = 30.77 min