we are given that Sheldon reads 5 pages every 15 minutes and Betty reads 10 pages every hour.
To determine the difference when both read 400 pages we need to determine how long it takes for each one to read one page. To do that we will find the quotient between the number of pages and the time it takes to read them.
For Sheldon we have:
For Betty we have:
Now we can find the time it takes them to read 400 pages by multiplying by the quotient we have found:
For Sheldon:
For Betty:
Now we find the difference of each times:
Now we convert the minutes into hours by dividing by 60:
Therefore, it takes Betty 20 hours more than Sheldon to read the 400 pages book.