Answer:
37/40
Explanation:
1. Find the LCM (Lowest Common Denominator, this allows us to actually add the two fractions together).
3/8 = 15/40
11/20 = 22/40
The numbers 8 and 20 both share the number 40 in this case. 
2. Add the numbers together, then simplify. 
15/40 + 22/40 = 37/40 
Fortunately for us, 37/40 is already simplified. 
There you go!