i have a couple of question for people of they are willing to help me please,

1) the equations connecting the running cost of a piece of apparatus,R,and the number of times it is used in one month,N, are

R=(3N + 15)

----------

8

and

N= 8R

----- - 5

3

use these equations to calculate

a) the value of R when N=27

b) the value of N when R= 42

c) the value of R when N=0

this is really getting to me please help thanks to anyone who can )