Dr. 1 sees all patients with diabetes or hypertension, Dr.2 sees the rest of the patients. Over the past two years, records show that Dr. 2 sees 55% of patients. Records also show that 20% of patients have diabetes and 40% have hypertension. What percentage of hypertensive patients have diabetes?

I'm not sure which of the probabilities formulas to use.
P(a)+p(b) -p(a and b)
P(a) * p(b)

Question

Dr. 1 sees all patients with diabetes or hypertension, Dr.2 sees the rest of the patients. Over the past two years, records show that Dr. 2 sees 55% of patients. Records also show that 20% of patients have diabetes and 40% have hypertension. What percentage of hypertensive patients have diabetes? 

I'm not sure which of the probabilities formulas to use. 
P(a)+p(b) -p(a and b) 
P(a) * p(b)