Emily has some five-dollar bills and some ten-dollar bills. She has 15 bills, and she has 120 dollars. How many five-dollar bills does she have?

Question

You have to yse a table chart

Answer 1

Let F = The number of $5 bills and T = the number of $10 bills.  She has 15 bills total, so:

 

F+T = 15                

T = 15-F

 

She has $120 total:

 

$120 = ($5)*F + ($10)*T

$120 = $5*F + ($10)*(15-F)         [Substitute 15-F in place of T]

120 = 5F + 150 - 10F

-30 = -5F

 

6 = F

 

Check:

T= 15-F = 15=6 = 9

 

$120 = ($5)*6 + ($10)*9

$120 = $30 + $90

$120 = $120

Check!

Answer 2

Hi Malaiyah,

 

Here is how to do it:

 

($10) x + ($5) y = $120

 

x + y = 15

 

x    y

10   5

 

15   0

14   1

13   2

12   3

11   4

10   5

9    6   !

8    7

7    8

6    9

5    10

 

 

First, it has to add up to 15 bills and number of $5 bills must be even because it has to end with 0.  If there are 2 $5 bills, it will add up to $140.  If there are 4 $5 bills, it will add up to $130.  And if there are 6 $5 bills, it will make up $120: $10 x 9 which is $90 and $5 x 6 which is $30.  As you add them together, you will get $120.  There you have it.

 

Good luck.

 

Tip of the day:  Multiples of 5s will only end with 5 or 0.  5 times any odd number will always end with 5 and 5 times any even number will always end with 0.

Answer 3

x = $5 dollar bills

 

y = $10 dollar bills

 

x + y = 15

 

5x + 10y = 120

 

Solve for y from the first equation and substitute into the second to get

 

5x + 10(15-x) = 120

 

5x + 150 - 10x = 120

 

-5x = -30

 

x = 6, so y = 9

 

There are six $5 bills and nine $10 bills.