Given the sum x - (x * 0.25) = y
how do I work out what x was if the only known number is y
If you multiply both sides of the equation by 4, you get x=4y.
No, you get
4x-x=4y
3x=4y
x=(4/3)y
I don’t. But perhaps reformatting the problem will make the answer more apparent: 25% is 1/4. So:
x - 0.25 * x = y
turns into
x - 1/4 * x = y
No you don’t. You get:
3x = 4y
x = (4/3)y
If the only known number is Y, you can’t do anything.
Y is what percent of what? Doesn’t go anywhere.
If you call the percentage P ( .25 in you equation from the OP) and that is the number you’re after then you have to know x.
(x-y)/x = P (y ≠ 25 percent of x)
But it is simpler to use y/x = P (What percentage of X is Y)
Instead of x - (x*25) = y Use x *.75 = y (y = 75% of x)
x – (x * 0.25) would be 0.75*x.
(You can either use your powers of algebra to combine like terms: 1x – 0.25x = 0.75x, or just think that if you subtract off 25% of whatever x is, you have 75% of it left.)
Then divide both sides by 0.75: x = y / 0.75
(which agrees with what others have said, since dividing by 0.75 = dividing by 3/4 = multiplying by 4/3)
Thanks all.
Turns out I’d guessed at the right modification before asking.
x = y * 1.333333333333
(which is the same as x= (4/3)y)
It wasn’t giving accurate figures (out by up to .1) but that’s because the thing I was using to check my figures was rounding down every time)