Algebra
solve these simultaneous questions via elimination method:
2x + 3y = 12
4x - 3y = 6

(2x + 3y = 12) x (-2)
(4x - 3y = 6) x 1

-4x - 6y = -24
4x - 3y = 6

You can cancel out the x values by adding the two equations together. 
(-4x + 4x) + (-6y - 3y) = (-24 + 6)
-9y = -18
y = 2

Solve for x now...
4x - 3(2) = 6
4x - 6 = 6
4x = 12
x = 3

Check... (x = 3, y = 2)
2(3) + 3(2) = 12 
6 + 6 = 12
12 = 12 <- this works! 

4(3) - 3(2) = 6
12 - 6 = 6
6 = 6 <- this works!

2x + 3y = 12 
2*3 6  x = 3 because 2 times 3 equals 6y = 2 because 3 times 2 equals 6. 6+6=12.
4(3) - 3(2) = 6\/        \/12       6      = 6  

(I'm copy-pasting from the work I just did on another tab. I know the page would reload)

The amount used= 13.04- 1.9 = 11.14