What is the answer to -4/5 + 3/20 =

First, find the LCD of the two denominators.
Multiples of 5: 5, 10, 15, 20
Multiples of 20: 20
The LCD is 20.
Second, make the denominators the same as the LCD. / Your problem should look like: 
Third, simplify so that the denominators are the same. / Your problem should look like: 
Fourth, combine the denominators. / Your problem should look like: 
Fifth, simplify. / Your problem should look like: 

Answer as fraction: 
Answer as decimal: -0.65