Answer:
No solutions
Step-by-step explanation:

Substitute:

This system has no solutions.
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Two at the top (side by side)
Two at the bottom ☝
One on right side
One on left side
And grandfather in the middle
If the maximum capacity of the tank is 84 then do 84/7 which is 12 and if it's 2/7 full then your answer is 24