Working independently, susan can perform a certain task in 6 hours and tom can perform it in 7 hours. if susan and tom work together at the task for 3 hours, at which point susan leaves, how many hours will it take tom to complete the task alone?

0.5 hr = 30 minutes

Explanation:

Let the total amount of work to be done = 42 units

Thus, efficiency of Susan = 42 units / 6 hours = 7 units/hr

Efficiency of Tom = 42 units / 7 hours = 6 units/hr

They worked together for 3 hours, which means that total efficiency = 7 + 6 units/hr = 13 units/hr

Thus, Amount of work done = 13 units/hr * 3 hr  = 39 units

Amount of work left which must be completed by Tom alone = 42 - 39 units = 3 units

Thus, Time taken by Tom:

Thus, Time taken by tom = 0.5 hr = 30 minutes

i have to go with a for this one

it should be d or b but i'm sure it's d