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How Much Is 30% or 40% Off? -- Calculating Percents in Your Head

Updated on November 12, 2016
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TR Smith is a product designer and former teacher who uses math in her work every day.

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Learning how to mentally calculate percentages, discounts, taxes, and tips are useful skills, even in this day and age when we have phones and calculators to do all the math for us. With practice, in less than the amount of time it takes to enter the math problem on your phone or calculator, you can calculate a percent increase or decrease in your head. This is especially useful when you can't use your technology and don't have pen and paper handy. This guide shows some methods for mentally calculating 30% off and 40% off discounts, which you will frequently find when shopping.

Mentally Calculate 30% Off (Method 1)

When the price of something is reduced by 30%, it's new cost is 70% of the original, since 1 - 0.3 = 0.7. Therefore, a quick way to compute a 30 percent discount in your head is to multiply the price by 0.7. This is equivalent to multiplying by 7 and then dividing by 10.

For example, suppose a washing machine is regularly priced at $498, but is on sale at 30% off. First, we round $498 to $500 to make the math easier. Next, we multiply $500 by 7, which produces $3500. Finally, we divide $3500 by 10, which gives us $350. Thus, the price of the washing machine after the discount is about $350.

Calculate a 30% Discount in Your Head (Method 2)

The second method for computing a 30% discount requires two steps but may be easier for people who have a hard time multiplying by 7. The first step is to multiply the original price by 0.3 (equivalently, multiplying by 3 and dividing by 10), and then subtract this number from the original price.

For instance, suppose a new dryer costs $472 and is on sale for 30% off. The first step is to multiply 472 by 3, which can be broken down into 400*3 + 70*3 + 2*3 = 1200 + 210 + 6 = 1416. Next, we divide this by 10, which gives us 141.6. We can round this to 142. Finally, we subtract 142 from 472, which gives us 330. So the cost of the dryer is about $330.

Calculate 40% Off in Your Head (Method 1)

When something's price is reduced by 40%, it now costs 60% of the original price, since 1 - 0.4 = = 0.6. This means you can calculate a 40% discount by multiplying the price by 0.6, or equivalently by multiplying by 6 and then dividing by 10.

If you find it difficult to mentally multiply by 6 directly, you can do it indirectly by multiplying by 2 and then multiplying by 3.

For instance, suppose a new stove is priced at $520 but is on sale at 40% off. First, 520 times 2 equals 1040, and 1040 times 3 equals 3120. Dividing this by 10 gives you 312. Therefore, the price of the stove after the discount is $312.

Mentally Calculate a 40% Discount (Method 2)

The second technique for calculating 40% off is to multiply the price by 0.4 and subtract this amount from the original price. Multiplying by 0.4 is the same as multiplying by 4 and dividing by 10, or dividing by 10 and then multiplying by 4. Multiplying by 4 is the same as doubling something twice.

For instance, suppose a mini fridge regularly costs $180 but is now on sale for 40%. First, we calculate 40% of 18, which is 2 times 2 times 18, which equals 72. Now, we compute 180 minus 72, which equals 108. Therefore, the mini fridge costs $108.

Tips for Calculating Percentages in Your Head

Here are a few tricks to make it easier to mentally compute percent discounts or percent increases like taxes and tips.

  • Round prices to the nearest $10 or $100. The final answer will be an estimate, but it won't be a bad estimate if the original price is close to a nice number ending in zero.
  • Remember that X% is the fraction X/100 and that X% off is the same as multiplying the original quantity by 1 - X/100.
  • When multiplying by a decimal, you can first multiply by a whole number and then divide by 10. E.g., multiplying by 0.6 is the same as multiplying by 6 and then dividing by 10.
  • Remember common decimal to fraction equivalents. 25% is one-fourth, 50% is one-half, 33% is nearly one-third, 67% is about two-thirds. 5% is one-twentieth, 10% is one tenth, and 20% is one-fifth.


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    Shasta Matova 2 years ago from USA

    I tend to use a calculator and not even attempt mental math, but I have found it to be very handy. These tips do help do the calculations in your head. I like that you have provided more than one way, in case one method works better than another in a particular situation.

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