Insulin and the sugar in the blood

Two people eat the same sweet breakfast, and the sugar in their blood starts at $170$ mg/dL. One has a pancreas that works well. The other makes too little insulin. You will write the rule that brings the sugar back, follow both people for four hours, and compare them. Then the program draws both curves.

The idea. In every half hour, insulin removes a share of the extra sugar above the normal level of $90$ mg/dL: the next level is sugar - strength * (sugar - normal).

Your tasks.

1. Make next_level return the level half an hour later, by the rule above.
2. Set above to the number of readings after the start at which the person with weak insulin is still above $140$ mg/dL.
3. Set below_time to the first time, in minutes, at which the healthy level is below $110$ mg/dL.