SOLUTION: A vampire enters a town and bites 2 person every night. The bitten 2 persons grows in to two big vampires and bites 4 persons the next night. These 4 vampires bites 6 people the ne

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The sequence is

2,4,6,8,10,...

since you're adding 2 to each number to get the next number.

So on the 5th night, 10 people are bitten. Add up all the numbers from 2 to 10 in that list to get 2+4+6+8+10 = 30

Don't forget to add the original vampire that started everything to get 30+1 = 31 total vampires on the fifth night.