hi
distances nouns , sums of money, periods of time ......only when considered as a unit :
50 years is the minimum sentence for that kind of crime
2 miles is too far for me to walk
20 dollars is a normal price to pay for this .
But
20 miles are separating those two cities
20 dollars have be found on the ground next the coffee machine
collective nouns aren't always plural as money , people......
hope it helps