We want to rotate the direction of polarization of a beam of polarized light through 90 by

Question:

We want to rotate the direction of polarization of a beam of polarized light through 90° by sending the beam through one or more polarizing sheets.
(a) What is the minimum number of sheets required?
(b) What is the minimum number of sheets required if the transmitted intensity is to be more than 60% of the original intensity?