13) B

Through the first sheet of glass, 80% of the light will pass through to the second sheet.

Through the second sheet of glass, 80% of the 80% will passs through.

80% of 80% = 0.8 x 0.8 = 0.64

0.64 = 64%.

However, out of the 80% that passed through the first sheet, the (0.15) light reflected from the second sheet will be reflected again (0.15) towards the second sheet. 80% out of this light which is reflected again will pass through.
So  extra light passing through = (0.8 x 0.15 x 0.15 x 0.8) = 0.0144 = 1.44%

64 + 1.44 = 65.44

To the nearest 1%, this will be 65%

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