Question

The human eye is most sensitive to light having a frequency of about 5.5

Answer 1

In this one you have as datum, the frequency of the wave. The frequency you were given is:

f=5.5x10^14 Hz

Then, a reference we can use to link this datum with the wavelength is:

c=?*f Being c=speed of light 3x10^8);?=wavelength;f=frequency, so:

?=c/f = (3x10^8)/(5.5x10^14)=545x10^-9=545n (nanometers)

Ok, now we have to change units of length.

1cm=10x10^6 nm
1.8cm=18x10^6 nm

(18x10^6)/545 = 33027.5 . It means that across the thumb fit 33027.5 times the length of the wave.