A farmer can plow a field in 12 hours by using one tractor. His son, using a different tractor, can plow it in 10 1/2 hours. How long will it take the farmer and his son working together to plow the field

In one hour farmer can plow 1/12 of field.

In one hour son acn plow 1/10.5 of field.

In one hour if both worked together can plow 1/12+1/10.5 of field.

1/12+2/21.

=(7+8)/84.

=15/84.

It will take 84/15 H for both of them if work together to plow field.

=5 3/5 H.