Question

Working alone, it would take Shandra 45 minutes to mow 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. 
Which best describes the reasonableness of their solution?

Answer 1

if you average the two times they take to mow the lawn you will get 1 hour. so on average it would take them 1 hour because kelly is a bit slow :)

Answer 2

Answer:

Their estimate is wrong, as they are taking almost half the estimated time to complete the work.

Step-by-step explanation:

Working alone, it would take Shandra 45 minutes to mow the lawn

To mow the same lawn, also working alone, it would take Kelly 1 hour and 15 minutes. In minutes it becomes 75 minutes.

If they work together, the girls believe it will take them 1 hour to mow the lawn.

Let 't' is the time taken to complete the work working together.

$\frac{t}{45}+ \frac{t}{75}=1$

$5t+3t=225$

$8t=225$

t = 28.125 minutes

We can see that the it will take 28.125 minutes if they are working together, but they believed that it will take them 1 hour to do so.

So, their estimate is wrong, as they are taking almost half the estimated time to complete the work.