Question 451511
First press prints {{{1/28}}} of the book in a minute while the second press does {{{1/70}}} of the book in a minute.

Working together they will print

{{{1/28 + 1/70}}} = {{{5/140 + 2/140}}} = {{{7/140}}} = {{{1/20}}}

Now we can make a proportion that {{{1/20}}} of the book both presses print in a minute and we want to find out how long it takes both presses to print one book:

1/20 book : 1 minutes = 1 book : x minutes

1/20 * x = 1 * 1

x = 1 * 1 / (1/20)

x = 20 minutes