Answer:
r = 5%
Step-by-step explanation:
Construction cost 800.000
# of barrels produced 10.000
Price per barrel $4
let the interest rate = r 
 
Equate the net present value = 0 
 
800000 = 10000 x 4/(1 + r) + 40000/(1 + r)2 + ....... 
 
800000 = 10000 x 4/r 
 
r = 5%