So the solution is given, the question is how to get those numbers above? Please show all steps.

3. A stock index is currently 990, the risk-free rate is 5%, and the dividend yield on the index is 2%. Use a three-step tree to value an 18-month American put option with a strike price of 1,000 when the volatility is 20% per annum. How much does the option holder gain by being able to exercise early? When is the gain made? 1513.18 0.00 1313.63 0 .00|| 1 | 1140.39|| 31.07 1140.39| 0.00|| 990.00 87.51 990.00 66.08 859.44|| 152.75 859.44 140.56 746.10|| 253.90 647.71 352.29 3. A stock index is currently 990, the risk-free rate is 5%, and the dividend yield on the index is 2%. Use a three-step tree to value an 18-month American put option with a strike price of 1,000 when the volatility is 20% per annum. How much does the option holder gain by being able to exercise early? When is the gain made? 1513.18 0.00 1313.63 0 .00|| 1 | 1140.39|| 31.07 1140.39| 0.00|| 990.00 87.51 990.00 66.08 859.44|| 152.75 859.44 140.56 746.10|| 253.90 647.71 352.29