Math
posted by Francesca .
How do I solve for negative modular arithmetic?
Here is an example: 45 mod 13 = 7, but how?

52 mod 13 = 0
45(52)=7 
You can add multiples of 13 until you get a positive number, example:
45+4(13)=45+52=7
or even
45+10(13)=45+130=85≡7 mod 13 
Thanks for your help!
Also, I am kind of confused about finding the value of x.
Example: 9x ≡ 1 mod 10
How do I solve this? 
how long will it take for a 10,000 investment to grow to 15,000if it is compounded continuously at 5 %