Answer:
Part A y = 15(1 + 0.4)^x.
Part B 434 after 10 days.
Explanation:
Part A.
21 = 15 ( 1 + r)^1
15 + 15r = 21
15r = 6
r = 6/15 = 0.4
So the equation is
y = 15(1 + 0.4)^x.
Part B.
So after 10 days x = 10
y = 15(1 + 0.4)^10
y = 434 to the nearest whole number.