Grace works as a salesperson at an electronics store and sells phones and phone accessories, and makes a fixed commission for each phone sale and a different fixed
commission for each accessory sale. Yesterday Grace sold 18 phones and 21
accessories and made a total commission of $204. Today she sold 6 phones and 14
accessories and made a total commission of $82. Determine the price of each phone
and the price of each accessory.

We can use a system of linear equations to solve this. I will use the variable P for phones and A for accessories.

Our first equation is: 18P + 21A = 204. This is because she told 18 phones and 21 accessories yesterday which got her $204.

Our next equation is: 6P + 14A = 82. This is because she sold 6 phones and 14 accessories today which got her $82.

We can use elimination to solve this system. We can multiply the equation 6P + 14A = 82 by -3. This is so we can avoid sign errors if we multiply by 3. Our new equation is -18P - 42A = -246.

Next, we add that equation to 18P + 21A = 204. This gives us -21A = -42. Once we solve for A, by dividing by -21, we get A = 2.

Next, we can plug A into our equation. I will plug it into the second one. 6P + 14(2) = 82. This gives us 6P + 28 = 82. Once we get 6P alone, we get 6P = 54. Solving for P gives us 9.

One phone costs $9, and an accessory costs $2.