Answer:
4900 Litres
Explanation:
First we need a common demoninator:
3/7=15/35
Then we subtract the two to figure out how much 420 Litres is:
15/35-12/35=3/35
3/35=420 litres
Divide amount by the numerater for thow much 1/x is.
1/35=140
and multiply by the denomenator to get a full number on your fraction, and therefore a full tank.
140 x 35 = 4900