We know that an hour has 60 minutes. She took 20 minutes to do one batch so we can see how many batches she makes with dividing 60 by 20. This would give us 3 batches per hour. Now we know she makes 8 frosted cookies per 20 minutes so we multiply 8 by 6 (2 hours as 20 min) and it would give us 48 frosted cookies
