I'm going to assume you mean:
13 * (3/4) + x = 7 * (1/4)
Let's go ahead and simplify the right side.
7 * (1/4) = 1.75
So now we have:
13 * (3/4) + x = 1.75
Now let's take care of the right side. Multiplication before addition.
13 * (3/4) = 9.75
9.75 + x = 1.75
To isolate x, we subtract 9.75 from both sides.
9.75 - 9.75 + x = 1.75 - 9.75
x = -8
Hopefully I spaced this well. \('-')/
It will take 40 hours for the water tank to become empty.
Explanation:
Divide 30 by 0.75 (equivalent to 3/4).
9. 130 = x + 137
answer x = -7
10. 9x-3 = 4 + 8x
9x = 7 + 8x
answer x = 7
