How to determine the Peak Factor of a multiple frequency sine wave of different amplitudes?
I'm sure that there's a difficult way to do this but I will give you a hint to visualise the resultant waveform and sketch it out. 

If you note that the \$ 5 \omega t \$ is an odd harmonic of \$ \omega t \$ and sketch it out you will see that the peaks coincide. This means that the peak voltage will be the sum of the individual peaks and you should be able to work this out in your head.

See if you can draw it and solve it.