Lets assume that the weights of five containers are B1, B2, B3, B4 and B5 pounds respectively. Also, B1 ≤ B2 ≤ B3 ≤ B4 ≤ B5. It is given that five containers of oil are weighed two at a time in all possible ways. It means that each of the container is weighted four times. Thus,
4*(B1 + B2 + B3 + B4 + B5) = (165 + 168 + 169.5 + 171 + 172.5 + 174 + 175.5 + 177 + 180 + 181.5)
4*(B1 + B2 + B3 + B4 + B5) = 1734
(B1 + B2 + B3 + B4 + B5) = 433.5 pounds..........(1)
Now, B1 and B2 must add to 165 as they are the lightest one.
B1 + B2 = 165...............(2)
Similarly, B4 and B5 must add to 181.5 as they are the heaviest one.
B4 + B5 = 181.5.............(3)
From above three equation, we get B3 = 87 pounds
Also, it is obvious that B1 and B3 will add to 168 - the next possible higher value. Similarly, B3 and B5 will add to 180 - the next possible lower value.
B1 + B3 = 168.............(4)
B3 + B5 = 180.............(5)
Substituting B3 = 87, we get B1 = 81 and B5 = 93
From 2 & 3 equations, we get B2 = 84 and B4 = 88.5
Hence, the weights of five containers are 81, 84, 87, 88.5 and 93 pounds.