Aqid ERPThree questions, one calculator
A percentage is a part of a hundred. Most everyday sums with one come down to three questions, and the calculator answers each: what is a given percent of a number; what percent one number is of another; and by what percent a number rose or fell.
- 15% of 200: multiply the number by the percent and divide by 100 → 30.
- 30 is what percent of 200: divide the part by the whole and multiply by 100 → 15%.
- From 200 to 250: the change (50) divided by the starting number (200), times 100 → a rise of 25%. From 250 to 200 the same sum gives −20%: a fall is measured against where it started, not where it ended.
A worked example: a price rise
A supplier moved a unit from SAR 48.00 to SAR 54.00. The rise is 6.00, which against 48.00 is a 12.5% increase. To pass the same rise on to a selling price of SAR 80.00, 12.5% of 80.00 is 10.00, so the new price is 90.00.
| Change: 54.00 − 48.00 | 6.00 |
|---|---|
| As a percent of 48.00 | 12.5% |
| 12.5% of 80.00 | 10.00 |
| New selling price | 90.00 |
Where the sum goes wrong
Two mistakes account for most wrong answers. A fall of 20% followed by a rise of 20% does not get back to the start: 100 → 80 → 96, because the second percent is taken of a smaller number. And percentage points are not percents: a rate that moves from 5% to 15% rose by ten points, which is a 200% increase in the rate.
The calculator holds every figure to six decimal places in fixed point and rounds the answer half up to two, so 12.5% of 80.00 is exactly 10.00, not 9.999999.