Q. 53.9( 64 Votes )

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Method 1:

Given: Temperature at 12 noon = 10oC

Change in temperature per hour = - 2oC

To Find: Time at which temperature is 8ºC below zero = -8ºC.

Temperature Difference = -8 - 10 = - 18ºC.

-2ºC change is in 1 hour
1ºC change will be in 1/2 hour.
18ºC change will be in 1/2 × 18 hours.
18ºC change will be in 9 hours.
So, the time at which the temperature is -8ºC = 12 noon + 9 hours = 9 p.m

Now, temperature at 12 midnight = 12 noon + 12 hours
Therefore, we need to find a change in temperature in 12 hours.

In 1-hour change is -2ºC, in 12 hours change will be 12 × -2ºC = -24ºC.

So, the temperature at midnight = 10ºC - 24ºC = - 14ºC

Method 2:
Given:
Temperature at 12 noon = 10oC

Change in temperature per hour = - 2oC

Therefore,

Temperature at 1:00 PM = 10oC + (-2oC) = 8oC

Temperature at 2:00 PM = 8oC + (-2oC) = 6oC

Temperature at 3:00 PM = 6oC + (-2oC) = 4oC

Temperature at 4:00 PM = 4oC + (-2oC) = 2oC

Temperature at 5:00 PM = 2oC + (-2oC) = 0oC

Temperature at 6:00 PM = 0oC + (-2oC) = - 2oC

Temperature at 7:00 PM = - 2oC + (-2oC) = - 4oC

Temperature at 8:00 PM = - 4oC + (-2oC) = - 6oC

Temperature at 9:00 PM = - 6oC + (-2oC) = - 8oC

Hence,

The temperature will be 8oC below zero at 9:00 PM

Now,

Change in temperature in 12 hours = - 2oC × 12

= - 24oC

Also,

At midnight the temperature will be = 10 + (-24)

= - 14oC

Hence,

The temperature at midnight will be 14oC below zero.

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