The correct option is B 420 litres
Capacity of 1 drum = 60 litres
Capacity of 7 drums = (7×60) litres
In order to multiply by a number ending in zeros, we first need to multiply the number as if it didn't have zeros, and then we just write all of the zeros the number had, on the right of the final answer.
We know, 7×6=42.
Then, 7×60=420.
Therefore, 420 litres of milk can be collected in 7 drums.