What is the formula to solve this

Question

Brett can plow out all the driveways on his street with his new​ four-wheel-drive truck in

4 hours. Using a snow blade on a lawn​ tractor, his neighbor can plow out the same number of driveways in 6 hours.

How long would it take them to do the work​ together?

Answer 1

Use B=Brett's Speed and N=Neighbor's Speed

4B=Total

6N=Total

Find both B and N with respect to the total.

B=Total/4

N=Total/6

Setting x=Time to complete together:

x(B+N)=Total

x(Total/4+Total/6)=Total

Create common denominators

x(3Total/12+2Total/12)=Total

x(5Total/12)=Total

Multiply by 12 on both sides

5x*Total=12*Total

Divide by "Total" to cancel out on both sides

5x=12

x=12/5 Hours=2.4 Hours

Answer 2