In order to get to the result, we need to see first how many inches on the map make up for one mile on the ground:
4/10 = 0.4
So we have 0.4 inches on the map for one mile on the ground.
Next we should take in consideration how much inches are there in one mile:
1 mile = 63,360 inches
Since in this case we have 0.4 inches for one mile on the ground, the amount of inches in a mile should be divided by 0.4 in order to get to the ratio:
63.360 / 0.4 = 25,344
So the ration on the map is 158,400.