Answer:
#3.) The initial value of  16 gal at  x = 5 minutes means that 16 gallons of water was present 5 minutes after the barrel started leaking.
#4.) Find how many minutes until the barrel is empty of water.
Let y = 0,  to solve for time x.
0 = (-2/5)*x + 18
(2/5)*x = 18
x = (5/2)* 18 = 5*9 = 45 minutes
Up to and after 45 minutes, the barrel is empty of water.
Step-by-step explanation:
#2.)  
minutes:      5, 10, 15, 20
water(gal):  16, 14, 12, 10
Find slope:    slope m = (14 - 16)/(10 - 5) = -2/5
y - 10 = (-2/5)*(x - 20)
y - 10 = (-2/5)* x  + 8
y = (-2/5)*x + 18
rate of change slope means that for every minute  2/5 gallons of water is lost
#3.) The initial value of  16 gal at  x = 5 minutes means that 16 gallons of water was present 5 minutes after the barrel started leaking.
#4.) Find how many minutes until the barrel is empty of water.
Let y = 0,  to solve for time x.
0 = (-2/5)*x + 18
(2/5)*x = 18
x = (5/2)* 18 = 5*9 = 45 minutes