step1=given

4/5 inch on the map repersents 90 miles

therfore

4/5 in =90 miles

step 2=also 

given

on a map two cities are nine and three-fifths inches apart

therfore

9 3/5=48/5

step3= now using concept of direct proportion

(48/5 *90) * 4/5

= 691.2 miles 

it is your required  number of miles between the two cities provided.