# 1 file and 1 notebook cost $10. 1 file and 1 pen cost $6. Each notebook costs thrice as much as a pen. How much does 1 file cost?

##### 2 Answers

A files costs

#### Explanation:

Let the following variables apply (with all values in dollars):

#color(blue)f# : cost of a file#color(green)n# : cost of notebook#color(magenta)p# : cost of a pen

We are told

[1]

[2]

[3]

From [3] we know we can substitute

[4]

Subtracting [2] from [4]

[5]

which implies

[6]

From [6] we know we can substitute

[7]

and, after subtracting

[8]

#### Explanation:

Let the cost of pen be

**From A & B,**

**Equating C & D,**

**From B,**