Answer
(a) $f_1=50Hz$ (b) $f_3=150Hz$
Work Step by Step
(a) The smallest change in frequency is 50 hertz, so it can be guessed that the fundamental frequency is 50 hertz. (b) Use the formula that $$f_n=nf_1$$ and substitute known values of $f_1=50Hz$ and $n=3$ to get a third harmonic of $$f_3=3(50Hz)=150Hz$$
