24 is the common denominator because it is the next smallest number that both 12 and 8 go into. 12 x 2=24 but you have to do the same thing to the numerator if you do something to the denominator so since we multiplyed by 2 now multiply 5 by 2 and get 10. Then the same for 3/8 but 8 goes into 24 3 times so we miltiply by 3, 3x3=9 so you get 9/24 and 10/24. Hope this helps have a great day!
