Explanation:
Since the new poison kills half the population every 2 months, we can say that the remaining half will survive for another 2 months. Therefore, after 2 months, the rodent population will be half of 1,000,000, which is 500,000.
After another 2 months, the remaining half of the 500,000 will survive, which is 250,000.
After another 2 months, the remaining half of the 250,000 will survive, which is 125,000.
After 6 months, the rodent population will be 125,000.
Since 12 months is six 2-month periods, we can repeat this process again. After another 2 months, the rodent population will be 62,500.
After another 2 months, the rodent population will be 31,250.
After another 2 months, the rodent population will be 15,625.
Therefore, after 12 months, the rodent population will be 15,625.