Nicole purchased a prepaid phone card for $50.25. Calls cost 25 cents a minute using this card. The credit, C (In dollars), left on the card after it is used for X minutes of calls is given by the following.
C=50.25 -0.25%
How much credit is left on the card after Nicole uses it for 30 minutes of calls?
Idollars
5
?

So if we do not know what 12 times 9 is then we can round 9 to 10 and get
12 times 10
most people can multiply by ten so this should be easy so
12 times 10=120

12 times 9=108
she was off by 12 so a 10% deviation error

another way to do it is you multiply 12 by 10 and get 10 12's
then you subtract 1  12 and get 9  12's