Solving money problems with decimals

0:00- [Interviewer] We're told Yonna bakes three batches of cookies. Each batch costs $4.75 to make. She sells 28 of the cookies at a farmer's market for $1.25 each. What is Yonna's total revenue? What is Yonna's profit? Pause this video, have a go at that. Okay, so let's just remind ourselves what revenue and profit are.

0:23Revenue is the total amount of money that you are bringing in, but that isn't necessarily the total amount of money that you make at the end of the day, because you have expenses. So profit is essentially revenue minus expenses. So revenue, revenue minus expenses is equal to profit.

0:49So let's figure out what the revenue is, what the expenses are, and then we can figure out the profit. So maybe I will do revenue in pink, so total revenue. So if we look over here, revenue, let's see, what is she selling? What money is she getting from other people?

1:06The batches of cookies, this looks like an expense. Three batches, each batch costs $4.75 to make. So this part right over here tells us our revenue. 28 cookies at $1.25 each. If I were to just do it, if I were to estimate it in my head, that's a little less than 30, a little more than $1, I would say it's gonna be a little bit more than $30.

1:30Let's see what happens when I actually multiply it out. So if I have 1.25 times 28, I am going to get 5 times 8 is 40, 2 times 8 is 16, plus 2 is 20, and then 1 times 8 is 8, plus 2 is 10, and then I'm not going to go to the 10s place, and let me cross these out.

1:582 times 5 is 10, 2 times 2 is 4, plus 1 is 5, and then 2 times 1 is 2. And then if I were to add all of this together, I get three, five, zero, zero, but we have to remember, we had two digits behind the decimal place in the numbers that we are multiplying, so that we need a total of two digits behind the decimal place right over here.

2:22So I got the $35, and that's pretty close to my estimation. And so the estimation is also useful when you're multiplying decimals, because sometimes people make a mistake, put the decimal, say, if they put the decimal one place different, it would be $3.50 or it'd be $350, and so the estimation is like, no, no, no, it should be around $30 or a little bit more than $30, so I like this from my revenue number.

2:44Now, what are my expenses going to be? Maybe I will do that in red. So we have three batches of cookies, each batch costs $4.75 to make. So if I do 4.75 times 3, I am going to get 3 times 5 is 15, 3 times 7 is 21, plus 1 is 22, 3 times 4 is 12, plus 2 is 14. We have a total of two digits behind the decimal, so $14.25 is our expenses.

3:20And so our profit is going to be $35 minus, so $35 of revenue minus the $14.75 minus the 14 or 14.25 of expense will give us our profit. And so, let's see, we could do a little bit of regrouping here. If I take one dollar from here, that's equal to 10 tenths, and if I were to take one of those tenths, that's equal to 10 hundredths.

3:5510 hundredths minus 5 hundredths is 5 hundredths. 9 tenths minus 2 tenths is 7 tenths, and then I had $4 minus $4 is zero, and then 30 minus 10 is 20. So there you have it, we have a profit of $20.75.