If the average price of four pairs of shoes is $50, and three of those pairs of shoes are priced at $35, $40 and $55, what is the price of the fourth pair of shoes?

The average price is calculated by dividing the sum of all prices with the number of items. 
average price= price1+price2+...priceN/N
In our case , we have four pairs of shoes.So, N=4.
We also know the average price. average price=$50
We also know the price of the first pair of shoes (price1), of the second (price2) and the third (price3). We need to find out Price4:
$50=$35+$40+$55+Price4/4
50*4=$35+$40+$55+Price4
$200=$130+Price4
Price4=200-130
Price4=$70

Im pretty sure it would be 6 purple rectangles.