Question

Working alone , it would take Sandra 45 minutes to now the lawn. TO mow the same lawn, also working alone , it would take kelly 1 hour and 15 minutes. if they work together, the girls believe it will take them 1 hour to mow the lawn

Answer 1

No. You don't take the average.  The formula is:

(worker 1 * worker 2) / (worker 1 + worker 2)

(45 * 75) / (45 + 75) =

3,375 / 120 = 28.125 minutes total time