Find the sum of the prime numbers less than 21

Apatient is given 50 mg dose of medicine the medicines effectiveness decreases every hour at a constant rate of 40% what is the exponential decay function that models this scenario how much medicine will be left in the patients system after 2 hours

The difference between tax advoidance and tax evasion

Find the center and the radius of the circle with the equation: x^2-2x+y^2+4y+1=0?