Answer :
We can substitute the expression for y from the first equation into the second equation, which just means setting the two equations to be equal:
35 + 5x = 100 - 8x
5x + 8x = 100 - 35
13 x = 65
x = 5
That's 5 minutes presumably, at which time each tank has 35+5(5)=60 units of water.