Some Algebra For The Day

High school math is an awesome gift, if you remember how to use it.

The thing is, I have R380 in eBucks and apparently qualify for a 20% discount. So I needed to figure out how much money I can claim to have, then activate the 20% to end up at R380. So I wrote this equation:

x - 0.2x = 380

The trouble is I forgot that x is actually a whole and taking away 0.2 of x leave you with 0.8x and 380 divided by 0.8 is 475, ie:

0.8x = 380
(4/5)x = 380 // convert decimal fraction to proper fraction
4x = 1900  // multiply both sides by 5
x = 475 // divide both sides by 4, and et voila

Simple, right? Except, as I said I forgot that x subtraction part. I was eventually helped by a cool website aptly named which did this form me:

x + -1(x * 0.2) = 380

Reorder the terms for easier multiplication:
x + -1(0.2x) = 380

Remove parenthesis around (0.2x)
x + -1 * 0.2x = 380

Multiply -1 * 0.2
x + -0.2x = 380

Combine like terms: x + -0.2x = 0.8x
0.8x = 3800

0.8x = 380

Solving for variable 'x'.

Move all terms containing x to the left, all other terms to the right.

Divide each side by '0.8'.
x = 475

x = 475

So, I should be able to pay for R475 item with R380. The tough part convincing a merchant that I have enough eBucks, well, technically.