A share portfolio consists of 100 shares "A" and 200 shares "B", which were acquired in t0 and have developed as follows

	t0	t1	t2	t3	t4	t5	t6
Kurs A-AG	220	224	236	221	218	225	220
Kurs B-AG	110	110	115	120	109	106	110

Please calculate the Value at Risk (VaR) 80 % confidence level using historical simulation for

a) Stocks "A" alone, b) "B" shares alone, c) Share portfolios consisting of "A" and "B" shares.

Why does the risk of the portfolio not correspond to the sum of the risks of the individual shares?