We are setting up virtual shopping tasks on an online survey. In each shopping tasks, we can only show up to 20 products on the screen. There are 80...

We are setting up virtual shopping tasks on an online survey. In each shopping tasks, we can only show up to 20 products on the screen. There are 80 products in total that they want to test. How many shopping tasks do we need to show so that on average, each product is shown 3 times throughout all the shopping tasks. ( Assume all products have been selected randomly and have equal chance of being selected)