Physics
It is easier to roll a barrel full of coal tar than to pull it because

more surface area of the barrel is in contact with the road when it is pulled
the rolling friction is much less than the sliding friction
the full weight of the barrel comes into play when it is pulled
coal tar is a liquid and it flows in the barrel when it is rolled

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Physics
It takes much longer to cook food in the hills than in the plains, because

due to low atmospheric pressure on the hills, the water boils at a temperature higher than 100oC and therefore water takes longer to boil
in the hills the atmospheric density is low and therefore a lot of heat is lost to the atmosphere
in the hills the humidity is high and therefore a lot of heat is absorbed by the atmosphere leaving very little heat for cooking
n the hills the atmospheric pressure is lower than that in the plains and therefore water boils at a temperature lower than 100oC causing an increase in cooking time

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