Answer:
500
Step-by-step explanation:
Let's see how many cardboards he sold in each month:
March: 350 units
April 350 + 50 = 400 units
May: 400 + 350 = 750 units
The average is the total of units he sold in the period we are considering, divided by the amount of months of that period.
Total: 350 + 400 + 750 = 1500
Amount of months = 3 (March, April and May)
So, the average is 1500/3 = 500