9m+20n refers to the total cost of buying 9 dictionaries and 20 science and technology stories;

Substitute m=50 and n=8.5 into 9m+20n,

9m+20n，

=9×50+20×8.5,

=450+ 170,

=620.

So the answer is: the total cost of buying 9 dictionaries and 20 science and technology stories; 620.