It seems like the question is asking for the frequency.
Given:
Time period (T) = 2.4 sec
Frequency (f) =?
We know that the formula for frequency is:
Frequency (f) = 1/time period (T)
= 1 / 2.4 s
= 0.42 Hz. is the frequency for this problem.
Heat from burning fuel warms the walls of the firebox section of the furnace in
A. a hot-water heating system.
B. a hot-air heating system.
C. a compressor compartment.
D. an evaporation system.