Lyle bought 3/8 pound of red grapes and 5/12 pound of green grapes. how much pounds of grapes did he buy

He bought 19/24 pounds of grapes.   

   3          5 
 + = 0
   8         12

In adding fractions. 
Step 1: make sure the denominators are the same. 8 and 12 are not the same. But both are factors of 24. 

8 * 3 = 24
12 * 2 = 24

Multiply both numerator and denominator of 3/8 by 3 and 5/12 by 2.

 3 * 3                5 * 2           9         10         
     +      = +  
8 * 3                12 * 2         24        24        

Step 2. Since the denominator is now the same, add the numerator and put it over the denominator.

9 + 10       19          
=   
   24          24

Step 3. Simplify the fraction.

19 

24   is already the simplified form.

Maybe the answer is 20mm. all i really did was just multiply, which is obvious.