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Problem of the Week
Problem A and Solution
Reading Randomly

Problem

Anya wants to get into the habit of reading for at least \(20\) minutes each day. She makes a plan to build up her reading gradually over several days. On Day \(1\), she reads for \(5\) minutes. After Day \(1\), she plans to read according to the following pattern:

If she follows the pattern, on which day is the first time Anya reads for at least \(20\) minutes? Justify your answer.

Solution

One way to solve this problem is to create a table to keep track of how long Anya reads each day.

Day Even/Odd Minutes Reading
\(1\) odd \(5\)
\(2\) even \(5 \times 2 = 10\)
\(3\) odd \(10 - 4 = 6\)
\(4\) even \(6 \times 2 = 12\)
\(5\) odd \(12 - 4 = 8\)
\(6\) even \(8 \times 2 = 16\)
\(7\) odd \(16 - 4 = 12\)
\(8\) even \(12 \times 2 = 24\)

If she follows the pattern, Anya reads for at least \(20\) minutes on Day \(8\).