Hi, 

it's about identifying the variable that caps the maximum potential revenue of a business. Very common examples are cashiers, tables, seats, ... In your example the bottleneck is the number of cashiers, because since each cashier can serve a max of 20 customers per hour (assuming you need 3 minutes to serve a single customer), you set the maximum revenue of the activity

I would like to suggest also cases in the platform to practice with:

• https://www.preplounge.com/en/consulting-forum/how-much-would-you-charge-to-clean-all-the-windows-in-seattle-4965
• https://www.preplounge.com/en/consulting-forum/market-sizing-milk-consumption-5087
• https://www.preplounge.com/en/consulting-forum/how-would-you-calculate-the-value-of-a-cow-4982
• https://www.preplounge.com/en/consulting-forum/estimate-number-of-traffic-lights-in-a-london-5692

Hope it helps,
Antonello