we know there are 60 minutes in an hour. To find 2/3 of an hour we divide 60/3 to find 1/3 and multiply by 2 so:
60/3 = 20
20*2 = 40 minutes (which is 2/3 of an hour) to do science
We do the same thing with the 40 minutes. To find 1/4 of 40 minutes we divide 40 by 4 and then multiply it by 3 to get 3/4 so:
40/4 = 10
10*3 = 30 minutes
Then subtract the times 40-30 = 10 minutes
It took her 10 more minutes to complete her science homework.
Hope that helps
