Three perfectly polarizing sheets are spaced 2 cm apart and in parallel planes. The transmission axis of the second sheet is $30 ^\circ$ relative to the first one. The transmission axis of the third sheet is $90 ^\circ$ relative to the first one. Unpolarized light impinges on the first polarizing sheet. What percent of this light is transmitted out through the third polarizer?
Source: Giancoli, Douglas C., Physics: Principles with Applications, 7th Edition, 2014.