The ratio of the volume that flowed out from tank A and B until tank B is emptied is shown below

A : B => 10 : 5     scenario 1

A : B => 5  : 10    scenario 2.

Both scenarios, Tank B is the same volume, as the question says leak until B is empty. So times 2 to scenario 1 and make both scenarios share common units ‘u’. 

A : B => 20 : 10   scenario 1

Water in Tank A can be represented by equation below..

A = 20u + 300    – >(1) All that flows out in scenario 1+ 300

A = 5u + 750    –  >(2) All that flows out in scenario 2 + 750

We now have a simultaneous equations.

(1) – (2)

0 = 15u – 450,   15u = 450

u = 30

A = 5u +750 = 900 ml ##

pls help