Question

@whpalmer4

steve bought 12 used CDs and DVDs . CDs cost $2 each and DVDs $3. He spent $31 not including tax. How many DVDs did steve buy ?

Answer 1

im thinking 7 right ?

Answer 2

Correct

Answer 3

:)

Answer 4

C cds and D dvds

C + D = 12
2C + 3D = 31

multiply first equation by -2, and add them together:

-2C - 2D = -24
2C + 3D = 31
--------------
0C + D = 7
D = 7

C + D = 12
C + 7 = 12
C = 5


Or, the "in your head" method. Assume all 12 are CDs. 12*2 = 24. That's 31-24=7 dollars short of what he actually spent. Each DVD costs 3-2 = 1 dollar more than a CD, so trading 7 CDs into DVDs bumps up the expenditure by 7 and gives the answer.

Answer 5

thank you !