Computation with Percentages

In problems involving percentages, it is usually easiest to convert to a fraction or a decimal. Let’s look at two examples. First, we’re going to calculate 40% of 62. Of, in math, means to multiply.

So, we’re doing 40% times 62. So, now, we need to convert 40% either to a fraction or to a decimal.


The following transcript is provided for your convenience.
I’m going to convert it to a decimal, and to do that, all I have to do is take my decimal and move it two places to the left. So, 40% as a decimal is 0.4, times 62. 0.4 times 62 is 24.8. So, 40% of 62 is 24.8.

You can use mental math to find percentages, and on this one, what we could do is find 10% of 62 first, and then multiply that times 4, since 10% times 4 would be our 40% of 62. So, to find 10% of 62, we would just move the decimal one place to the left, so that would be 6.2, and then we want to do that 4 times, so we just multiply that times 4. So, 6.2 times 4, which would again, give us 24.8, and this you could do a little easier in your head.

Let’s look at one more. Oftentimes, percentages are used to calculate the tip when you got to eat a restaurant. So, here we’re going to calculate a 20% tip on a $28.75 meal. So, we want to know what is 20% of $28.75, since that’s how much we need to leave them. 20% of how much our total bill was.

So, again, of is multiplication, and we want to convert our percent to a decimal, so move it two places to the left, so we’re really doing 0.2 times $28.75, and 0.2 times $28.75 is $5.75. So, we would need to leave them a $5.75 tip.

What would our total bill be? Well, our total bill would be how much our food costs plus how much we’re leaving in as a tip. So, $28.75 plus $5.75. 5 plus 5 is 10, write your 0, carry your 1. 7 plus 7 is 14, plus 1 is 15, carry your 1, bring down our decimal. 8 plus 1 is 9, plus 5 is 14, write your 4, carry your 1. 2 plus 1 is 3, so our total bill would be $34.50 with the tip included.