Question 824305
Call the tanks A and B.  Originally A=B.  Say the water was pumped from A to B.<P>
That means subtract 3000 liters from A (A-3000) and add 3000 liters to B (B+3000).  B+3000 is 6 times A-3000.<P>
6(A-3000) = B+3000 ---> 6A - 18000 = B + 3000.<P>
Remember, that A = B.  So substitute B into the equation for A.<P>
6B - 18000 = B + 3000<P>
5B = 21000<P>
B = 21000/5 = 4200 = A.<P>
Both tanks originally held 4200 liters.  After 3000 liters were pumped from A to B:<P>
A held 1200 and B held 7200.  6*1200 = 7200