Calculate what is the temperature of the air overboard of an airplane flying at an altitude of 5 km, if at the earth’s surface it is 18 ° C

For the convenience of solving the problem, first we translate the height at which the plane flies from kilometers to meters.
Since 1 kilometer is 1000 meters, we get:
5 * 1000 = 5000 meters.
We write the value of the vertical temperature gradient. It is equal to 0.6 ° C per 100 meters.
This means that the air temperature decreases by a given value when rising for every 100 meters.
In this case, we get:
5000 * 0.6 ° C / 100 = 30 ° C.
Since this number shows the temperature difference, then overboard will be:
18 – 30 = -12 ° C.