1) The common approach for this type of the questions is to calculate the rate of a unit. In other words "how much of the parking lot does a snow plow clear in 1 hour"?

We start with the fact that 5 snow plows clear the parking lot in 12 hours. On the diagram we show a rectangular parking lot, but the shape of the parking does NOT affect the reasoning.

2) Since 5 snow plows clear the iced parking lot in 12 hours, then each of the snow plows clears 1/5 of the parking lot.

3) Since a snow plow clears 1/5 of the parking lot in 12 hours, then in 1 hour it clears (1/5)/12 of the parking lot.

(1/5)/12 = 1/(5 × 12) = 1/60

Now we have a rate for one snow plow. That's the key moment to all such questions.

4) Since a snow plow clears 1/(5 × 12) of the parking lot in 1 hour, then 6 snow plows clear 6 × 1/(5 × 12) = 1/10 of the parking lot in 1 hour.

5) Since 6 snow plows clear 1/10 of the parking lot in 1 hour, then to clean the whole parking lot it will take them 1 / (1/10) = 10 hours. The correct answer is A.

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This is the basic logic that stands behind all of questions of this type. Once you understand it, you can take shortcuts, like using the whole formula at once:

new time = 1 / ([the rate of 1] × 6)

new time = 1 / (1 / (5 × 12) × 6) = 1 / 0.1 = 10