<span>20.28% is the answer</span>
Answer:

Step-by-step explanation:
Hello
let's say that the price of the book was x
the price after a 20% discount is x - 20%*x = x*(1-20%)=x*(1-.20)=0.8*x
and this is $72 so we can write that
0.8*x=72
and then divide by 0.8 both parts
x = 72/0.8=90
So the list price value of the book is $90
and we can verify as 90 - 20%*90 = 90 - 18 = 72
Hope this helps
Answer:
b=2
Step-by-step explanation:
we have
9x+12y=21 -----> equation A
6x+8y=7b ----> equation B
we know that
If the system of equations have an infinite number of solutions then the equation A must be equal to the equation B
Multiply equation B by 1.5 both sides
1.5*[6x+8y[=7b*1.5
9x+12y=10.5b ----> equation C
Compare equation A and equation C
9x+12y=21 -----> equation A
9x+12y=10.5b ----> equation C
For the equations to be equal it must be fulfilled that
21=10.5b
solve for b
b=21/10.5
b=2