Hi,
I just wanted to contribute to solving this question in an easier way, in my opinion.
I chose to compare machine A’s rate in two scenarios by using the given number of additional number of machine B from Quantity B (21). Since it turns out that adding 21 more machine B makes machine A’s rate lower, it shows us that adding 21 more machine B produces more than triple of the number of aluminum cans when machine A’s rate is equal.
Hope this helps!