Explanation:
10 pages in 5 hours.
our focus is on pages (how long for 20 pages).
so, if we don't see the ratio right away, let's norm the original relationship to 1 page :
10 pages in 5 hours
so,
1 page in 5/10 = 0.5 hours = 1/2 hours
and now : how long for 20 pages ?
1 × 20 pages in 1/2 × 20 hours
20 pages in 10 hours.
formally, we are dealing with a ratio, which is a fraction :
10 pages in 5 hours = 10/5
to keep the value of the fraction, in order to change one of the 2 values to a target value (like 10 pages to 20 pages in our case), we need to multiply the other with the same factor :
10/5 × 2/2 = 20/10
so, again, this tells us 20 pages in 10 hours.