Under the quota,
The willingness to pay of an individual tell us that what price he is ready to pay for quantity in the market. There is quota in the market, so Cindy and Carl cannot buy more than quota amount. Thus, Last unit that Cindy purchase is priced $0.6. The last unit Carl purchased at $0.3.
Under the tax,
Cindy will use 30 plastic bags and Carl will use 10 bags. We can see this into the diagram under tax. Cindy's demand curve cut tax line at 30 bags and Carl's demand curve cuts tax line at 10 bags.
The willingness to pay for the last bag used for Cindy is $0.4 and for Carl $0.4