Blue cassests hold 45 min. of music green cassets hold 55 min. of music if niko has 16 cassets containing 830 min. of music how many are blue? and how many are green?

Let the number of Blue casset be b and that of green be g.
Thus the total number of cassets will be:
b+g=16
hence
g=16-bi

The total amount was:
45b+55g=830..ii
plugging i in ii we get:
45b+55(16-b)=830
45b+880-55b=830
simplifying 
45b-55b=830-880
-10b=-50
thus
b=5
hence
g=16-5=11
number of blue cassets was 5 and that of green was 11