The temperature change in a chemistry experiment was -2°c every 30 minutes. the initial temperature was 6°c. what was the temperature after 4 hours?

Initially it was 6°,

30 minutes later it went down to 6 - 2, 
30 minutes later it went down to 6 - 2 - 2

let's check the change per intervals of 30 minutes

so after 1 interval of 30 minutes, it went down to 6 - 2

after 2 intervals of 30 minutes, it went down to 6 - 2 - 2, or just 6 -2(2)

after 3 interval of 30 minutes, it went down to 6 - 2 - 2 - 2, or just 6 -2(3)

after "x" intervals of 30 minutes, it went down to 6 - 2(x).

now, how many 30 minutes intervals are there in 4hours?  well, 8.

so after 4 hours, x = 8, thus it went down 6 - 2(8) degrees Celsius.

Logical equivalent of the give question is its contrapositive