A library is having a used book sale. All hardcover books have the same price and all softcover books have the
same price. You buy 4 hardcover books and 2 softcover
books for $24. Your friend buys 3 hardcover books and
3 softcover books for $21. What is the cost in dollars of a
hardcover book?

 It should cost $5

Step-by-step explanation:

First you have 4 hardcover and 2 softcover, so 4x+2y= $24 and for your friend it would be 3x+3y= $21.

You have

(3x+3y)2  = 72

(4x+2y)3  = 42

Then subtract them which is 30 you would divide by 6 since 3x2=6

Which gets you to 5.

answer: 80,000

step-by-step explanation:

the population doubles every day.

10,000*2=20,000 day 4

20,000*2=40,000 day 5

40,000*2=80,000 day 6

i believe the answer is c.