plural forms - Why "was" not "were" in "Nearly £20 was taken from my bank account"

I've always said "$100 were taken" not "$100 was taken" because I thought $100 is plural. Could you explain why "was" not "were"? Any other helpful notes about the issue would be appreciated.

Nearly £20 was taken from my bank account

Answer

Because phrases that indicate the amount of sum, time, distance, weight, temperature, etc. are treated as singular:

10 million pounds is a lot of money.

= This sum is a lot of of money.

50 liters of petrol fills my car.

= This quantity fills my car.

Five kilometers is a long way to walk.

= This distance is a long way to walk.

Fifty degrees is a very high temperature.

= This temperature is a very high temperature.

And so on.