A lunch costs $40. The tip is 15%. What is the total cost? Step by Step

Step by step

The question

Question: A lunch costs $40.

The tip is 15%.

What is the total cost?

There are four choices.

First we solve it, the normal way.

Then we pick.

1 Find the tip

Percent means out of 100.

So 15% is 15 divided by 100: 0.15.

The point moves 2 places to the left.

15% = 15 ÷ 100 = 0.15

The tip is 15% of $40.

"Of" means times: 0.15 times $40.

To multiply decimals: first 15 times 40, as whole numbers.

That is 600.

Then count the decimal places: 2, the digits after the point in 0.15.

So it is 6.00.

15 × 40 = 600

2 decimal places: 6.00

So the tip is $6.

tip = 0.15 × $40 = $6

Careful!

15% is not $15.

It is 15% of the price.

Careful: not $40 + $15 = $55 ✗

2 Add it on

The tip is added to the price.

$40 plus $6 is $46.

total = $40 + $6 = $46

So the total is $46.

3 Check

Let's check another way: we pay 100% plus 15%: 115% of the price.

1.15 times $40 is $46.

The same.

So the answer is right!

100% + 15% = 115%: 1.15 × $40 = $46

4 Pick the choice

Our answer is total: $46.

That is choice C.

C) $46

5 Why not the others

A is $6.

Wrong: that is the tip alone, not the total.

A) $6 ✗ that is the tip alone, not the total

B is $34.

Wrong: took 15% off: a tip is added.

B) $34 ✗ took 15% off: a tip is added

D is $55.

Wrong: 40 + 15: added 15 dollars, not 15%.

That is the Careful mistake from before.

D) $55 ✗ 40 + 15: added 15 dollars, not 15%

Let's look back.

First: find the tip.

Then: add it on.

Then: check.

Then: pick the choice.

Last: why not the others.

Answer