Whats the error? jonah says a common denominator for 3/4 and 2/5 is 9 whats the error. whats the answer

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

Four and five do not meet at nine. they meet at another number.
the answer is X/20

Hope I helped!   ;-)