3/4 inch represents 1/8 mile, that means:
1/8 mile represented by 3/4 inch
1 mile will be : (3/4) inch ÷ (1/8)
= 3/4 × 8/1 = 6
1mile will be represented by 6 inches
So on the map it is 6 inches per mile
You divide 10 by 5 is two so 19 decided by five is 3.8