Finding the optimum temperature

Measurements of an enzyme's rate at seven temperatures are listed. You will find the optimum temperature, turn every rate into a percentage of the highest rate, and list the temperatures at which the enzyme works at half its best rate or better.

The idea. The optimum is the temperature with the highest rate. A percentage is $100 \times \text{rate} \div \text{highest rate}$.

Your tasks.

1. Complete optimum(rates) so that it returns the temperature with the highest rate.
2. Fill the dictionary percent with each temperature and its rate as a percentage of the highest rate.
3. Set working to a list of the temperatures whose rate is at least $50\%$ of the highest.