Answer: 5
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Explanation:
- third term = x
- fourth term = 2(third term)+1 = 2x+1
- fifth term = 2(fourth term)+1 = 2(2x+1)+1 = 4x+2+1 = 4x+3
Set the fifth term equal to 23. Solve for x.
fifth term = 23
4x+3 = 23
4x = 23-3
4x = 20
x = 20/4
x = 5
The third term is 5. This leads to:
- fourth term = 2(third term) + 1 = 2*5+1 = 10+1 = 11
- fifth term = 2(fourth term)+1 = 2*11+1 = 22+1 = 23
We've confirmed the answer.
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Here's another way to get the answer:
The process "multiply previous term by 2, then add 1" can be reversed to: "subtract 1 from the previous term, then divide by 2". We undo each of the operations mentioned and we do so in reverse order.
So,
- fifth term = 23
- fourth term = (fifth term-1)/2 = (23-1)/2 = 22/2 = 11
- third term = (fourth term-1)/2 = (11-1)/2 = 10/2 = 5