Pam typed 2.5 pages in 15 minutes. at that rate, how many minutes will it take her to type 23 pages?

Well, first of all, you have to compare;
15 minutes=2.5 pages
              x=23 pages
   Then you cross multiply. By the way, i replaced the unknown with x. Anyway, x multiplies 2.5 and 23 multiplies 15. So that will be;
2.5x=345
  Then you divide both sides by 2.5. This is done to remove all that is attached to x to find x
2.5x/2.5 = 345/2.5
  Therefore, 345 divided by 2.5 is 138. So;
x=138
  It will take 138 minutes to type 23 pages. HOPE I HELPED. 

Do the inverse operation