In a population, patients who have high cholesterol also have high blood-pressure (BP). Some patients with high BP also have diabetes. There are no patients who have both high cholesterol and diabetes. Furthermore,
- the total number of patients with at least one of these conditions is $75$,
- the number of patients with high cholesterol is $10$,
- the number of patients with high BP is $45$, and
- the number of patients with only high BP and no other conditions is $20$.
Then the number of patients who have both diabetes and high BP is $\_\_\_\_$
- $0$
- $15$
- $20$
- $10$