First, I would organize your information into equations. So, the first sentence would be:
d = 3 + t d = disk and t = tape
Second and third sentence would be the equation:
5 d + 2 t = $120
To find the price of the disk, you would follow these steps.
5 (3 + t) + 2 t = 120 substitute d, with the equation that has a total, for
(3 + 2), then solve for t
15 + 5 t + 2 t = 120 distribute
15 + 7 t = 120 add like terms
7 t = 105 subtract from both sides
t = 15 divide from both sides
Then you substitute 15 in the first equation.
d = 3 + t substitute
d = 3 + 15 add
d = 18
So, d = 18 and t = 15. To check this, you just need to substitute the right variables.
1) 18 = 3 + 15
18 = 18
2) 5 (18) + 2 (15) = 120
90 + 30 = 120
120 = 120
When checking, we can see that the correct answer is $18 for the price of the disk.
Answer: $18