Y0ou find a common denominatior, in this case is 15.... then multiply whatever u had to get the denominator 15 times the top number, so 3/5 + 10/15 = 13/15
Explanation: So you have 60 minutes in an hour and if you divide that in 4 you get 15. And for the last step you add 15 three times or just multiply 15*3 and get 45. Hope this helps :D