25. (2004 National College Entrance Examination Hubei Paper) A certain emergency event has a probability of 0.3 of occurring if no preventive measures are taken. If it occurs, it will cause a loss of 4 million yuan. There are two independent preventive measures, A and B, available. The costs of adopting measures A and B alone are 450,000 yuan and 300,000 yuan, respectively, and the probabilities of the emergency event not occurring after adopting the corresponding preventive measures are 0.9 and 0.85, respectively. If the preventive plan allows for the adoption of measures A and B alone, in combination, or not at all, determine the preventive plan that minimizes the total cost. (Total cost = cost of adopting preventive measures + expected value of losses from the emergency event.)
25. (1) Without taking preventive measures, the total cost, which is the expected loss, is $400 \times 0.3=120$ (ten thousand yuan);
(2) If only measure A is adopted, the preventive cost is 45 ten thousand yuan, and the probability of an emergency occurring is $1-0.9=0.1$, the expected loss is $400 \times 0.1=40$ (ten thousand yuan), so the total cost is $45+40=85$ (ten thousand yuan);
(3) If only measure B is adopted, the preventive cost is 30 ten thousand yuan, and the probability of an emergency occurring is $1-0.85=0.15$, the expected loss is $400 \times 0.15=60$ (ten thousand yuan), so the total cost is $30+60=90$ (ten thousand yuan);
(4) If both measures A and B are adopted, the preventive cost is $45+30=75$ (ten thousand yuan), and the probability of an emergency occurring is $(1-0.9)(1-0.85)=0.015$, the expected loss is $400 \times 0.015=6$ (ten thousand yuan), so the total cost is $75+6=81$ (ten thousand yuan).
In summary, adopting both measures A and B is the best option.