Algebra Demystified 2nd Ed

328 algebra De mystif ieD

7. Let x = number of times Sharon uses her skates
   The cost to rent skates is 3x.
   60 3
   60
   3
   20 20
    ( )

< 

< 

<  or > 

x

x

xx

Sharon would need to use her skates more than 20 times to justify

purchasing them instead of renting them.

8. Let x = number of kilowatt-hours used per month
   28 + 0.07x = monthly bill
   28 007 210
   28 28
   007 182
   182
   007

 .

. 

.

 ≤+

––

≤ 

≤ 

≤

x

x

x

x 2600

The James family can use no more than 2600 kilowatt-hours per month

in order to keep their electricity costs within their budget.

9. Let x = amount spent at the store annually
   0.05x = extra 5% purchase charge per year
   40 005
   40
   005
   800 800

 .

.

≤ 

≤ 

≤  or ≥ 

x

x

xx()

A shopper would need to spend at least $800 per year to justify the

$40 annual fee.

10. Let x = annual sales level
    0.08x = annual commission
    15,000 + 0.08x = straight annual salary plus commission

15 000 00825000

15 000 15 000

00810

,. ,

,,

. 

 + ≥

––

≥ 

x

x ,,

,

.

 ,

000

10 000

008

125 000

x

x

≥ 

≥