Since Tank C has the smallest amount of water, let u be the amount of water in Tank C.

Tank  C     [u]

Tank B      [u][u][7]

Tank A      [u][u][7][u][u][7]

Total = 147   

From the model diagram,

7u + 21 = 147

u = 126/7  = 18

Hence Tank A has 4u+14 = 4(18)+14 = 86 litres

Suppose Tank C has u litres.
Then Tank B has 7 + 2u (7 more than twice of Tank C).
And Tank A has twice of B so it has 14 + 4u.

Total is 21 + 7u = 147.
So u = (147 - 21)/7 = 18

So May poured 14 + 4 x 18 = 86 litres into Tank A.

If you prefer block diagrams, you can draw this:
C: [u]
B: [u][u][7]
A: [u][u][7][u][u][7]

Thank you! I got it!