Answer:
350 - 170 - 120 = r
Explanation:
Identify the variable:
r = number of red pens
The total amount of pens ordered is 350 when adding all three colors of pens. This means we must add all of the colored pens to find the total number of pens, 350.
Write this as an equation:
170 + 120 + r = 350
Rewrite the equation to find r:
350 - 170 - 120 = r