## What is simple rule of three?

The** rule of three** it’s an old mechanical method of solving problems based on proportions, which we can do quite easily. It is a basic mathematical tool used to quickly solve problems involving a directly proportional relationship between two variables.

## How is a percentage calculated with the simple rule of three?

Too easy. If, for example, in January of last year we sold 546 t-shirts and in January of this year we sold 748 t-shirts, we would be interested in knowing the percentage growth in t-shirt sales. Or put another way, what is the difference in sales.

So if 546 t-shirts are 100 percent of our sales for the month, the simple rule of three says that if 546 is 100%, what percent is 748 of 100%? We will have to multiply 748 x 100 and divide it by 546 by simple rule of three:

**[Si b es el porcentaje de a, el porcentaje de c es (c*b/a)]**

The result gives us 136.99. In January we sold 136.99% more t-shirts compared to January of last year. To know the percentage of variation using the data from the simple rule of three, the calculated percentage must be subtracted from this number, that is, 100.

The percentage increase in t-shirt sales between the two months of January is **36.99%**.

The equation is the following:

## 5 examples of simple rule of three to calculate percentage

If a bottle of milk was worth $2 last year and this year it costs $2.38, how much has it increased or what is the percentage variability of its price? If 2 is 100%, 2.38 is = 2.38 *100 /2 = 119 – 100 = **19% is the percentage increase**.

If I earned $100,000 gross last year and deducting government taxes, I would have $82,000 in hand. This year with the same gross salary I have $76,000 left in hand; what was the percentage loss in take-home pay? If 76,000 is 100% of what I have now, the percent change is = (76,000*100/82,000) – 100 = –**7,317% ****is the percentage decrease in my take-home pay**

If 12,560 runners participated in the marathon and 11,489 reached the finish line, the rest dropped out before arriving. What percentage of participants managed to reach the goal?

If 12,560 is 100% of those who participated, to know the percentage of those who reached the goal, you must calculate: (11,489*100/12,560) = **91.47% is the percentage of participants who reached the goal**.If a restaurant has 145 tables in total and a waiter serves 24 of those tables, what percentage of tables in the entire place does he serve? By rule of three: If 145 is 100, 24 is: 24×100/145 = **16.55% of tables**.On vacation I took 18,000 pesos to spend; I spent $5,360 on food. What percentage of my vacation money did I spend on food? You have to calculate: 5,360 x 100/18,000 =

**29.78% I spent on food, almost a third**.