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Life probability question watch

1. Q. The number of survivors to age x is given by the function: .
Calculate the probability that a life aged 21 will die after attaining age 40 but before reaching age 57.
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A. P(A life aged 21 will die after attaining age 40 but before reaching age 57) = (No. dying between 40 and 70)/(No of survivors to age 21)

l_40 = (121-40)^1/2=9
l_57 = (121-57)^1/2 = 8
l_21 = (121 - 21)^1/2 = 10

P(A life aged 21 will die after attaining age 40 but before reaching age 57) = (9-8)/10=0.1
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That's how my teacher done it. I'm trying to work out how he done it and if there is another way to do it. Any help appreciated.
2. Take age 21 to be your starting point. Number of people alive at the start is therefore 10. How many of these people die between ages 40 and 57? 1 person.
So the probably that someone dies between 40 and 57, given the starting point is age 21, is simply 1 divided by 10.
3. (Original post by obaydi)
Take age 21 to be your starting point. Number of people alive at the start is therefore 10. How many of these people die between ages 40 and 57? 1 person.
So the probably that someone dies between 40 and 57, given the starting point is age 21, is simply 1 divided by 10.
Thanks

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