1. Ans: 100/(1+0.05) + 100/(1+0.08)^2
Work the arithmetic out yourself for the next step. 
2. 5 annual payments of $50 discounted at the interest rate of
10% per annum:
Ans: 50/(1+0.1) + ... + 50/(1+0.1)^5
Rationale: Investor can use this same sum of money and re-invest
it at the same rate of 10%, giving him the same amount of money
after 5 years, therefore making him indifferent.
Work the arithmetic out yourself for the next step.
Edit: Typos