2 Answers

3/5+9/10= 1 1/2

covert 3/5 to 6/10

6/10 +9/10=15/10................1 1/2

The denominators must be the same when you add or subtract fractions.  3/5 and 9/10 have different denominators (5 and 10).  When that happens, you must find the lowest common multiple of both denominators.  In other words, what is the smallest number of which BOTH 5 and 10 are factors.   Let's think really hard.  1 x 10 = 10    5 x 2 = 10.   Let's use 10 as the denominator!!!!  (EZ)
Since multiply /5  x 2 to get to 10, you must also multiply the numerator (3) by 2 also; this keeps your fraction 3/5 equal.   6/10 is the same as 3/5.   You multiply 10 by 1 to get 10, so you multiply 9 by 1 also.  9/10  
Now you have 6/10 and 9/10.   (Think of two pizzas, cut into 10 pieces each.  You have 6 pieces of one left and 9 pieces of the other.  How many total pieces of pizza do you have).  6/10 + 9/10 = 15/10     When the numerator is larger than the denominator, you have what is called an "improper fraction".  FIX IT: 
1.  Divide the numerator (15) by the denominator (10).  You will get 1.5  or 1/5/10 or 1 1/2