Question

Predict the total cost if you bought 20 books.
Books Bought 1 3 5 7
Total Amount Spent $ 21 39 57 75
a)$192
b)$420
c)$200
d)$310

Answer 1

Every two books more you have to pay $18 more. So each book is $9, except the first, which was $21. 21+19. 9 equals A

Answer 2

Ok, sooo
We start out with an initial amount of 21 right?
So the starting point for us is $21

The data shows not every book, but every two books
It is not every three books because it starts with 1, not 0
1 + 2 = 3
3 + 2 = 5
etc.

So, from $21 to $39, how many dollars did we increase
Same with 3 to 5
From $39 to $57, how many dollars did we increase?

Answer 3

you increase $18 every two books right?

Answer 4

Yep!
So since it is every two books, you divide by 2
So that would mean how much is each book if each book is $18?

Answer 5

$9

Answer 6

Yep :)

Now we want to find the total amount she spent
SInce the first book was at an extra amount we'll set that one aside
To find the amount she spent, we would need to multiply 9 x 19
The number is 19 and not 20 because we set aside that first payment of $21

So, what is 9 x 19?

Answer 7

171

Answer 8

Perfect c:

Now you just add the beggining payment to that, which was the $21 we started with

So 171 + 21 = ?

Answer 9

192

Answer 10

And that's your answer c:

Great Job!

Answer 11

thx!!