Late for work

What average speed should Ms Fanthorpe drive at to arrive at work on time?
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Problem



Every day, Ms Fanthorpe leaves home at 8:30 and drives to work.

If she drives at an average speed of 20 miles per hour, she arrives 3 minutes late. If she drives at an average speed of 30 miles per hour, she arrives 3 minutes early.

What average speed should she drive at to arrive on time?

This problem is adapted from the World Mathematics Championships