-4/5 + 3/20

Well, this site does have a really good calculator:) http://web2.0calc.com/

Enter: -4/5 + 3/20

$${\mathtt{\,-\,}}{\frac{{\mathtt{4}}}{{\mathtt{5}}}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{3}}}{{\mathtt{20}}}} = {\mathtt{\,-\,}}{\frac{{\mathtt{13}}}{{\mathtt{20}}}} = -{\mathtt{0.65}}$$

And if you prefer to do the problem by hand:

Convert (-4/5) into a fraction over 20 by multiplying it by (4/4)

= -16/20

Then add 3/20 to -16/20:

= -13/20