Jonah is incorrect because 3/4 and 2/5 don't have a common denominator of 9.Jonah probably added 4 and 5 together but that isn't the common denominator. You can check by doing this:
4,8,12,16,20,24,28,32,36,40
5,10,15,20,25,30,35,40,45,50
As you can tell 3/4 and 2/5 common denominator could be 20, 40 and many more but the more reasonable denominator would be 20.
3 times 5 = 15 2 times 4= 8
- -
4 times 5 = 20 5 rimes 4=20
You have to multiply the numerator the same number that you multiplied it with the denominator to get 20
