Final answer:
The rate at which angel practices the piano is 33 minutes per day.
Step-by-step explanation:
In this problem, we are given the equation m = 33d, where m represents the total number of minutes angel practices the piano and d represents the number of days. To find the rate at which he practices, we need to find the change in minutes per change in days. This can be done by finding the slope of the equation, which is the coefficient of d, 33.
Therefore, the rate at which angel practices the piano is 33 minutes per day.