As my autumnal birthday approaches I like to collect leaves! A little bizarre perhaps, but I enjoy it!

Starting on the first day of the month I collect 1 leaf, on the second day I collect 2 leaves, the third day I collect 3 leaves, and so on.

On my birthday, I will have collected 276 leaves altogether. Which day of the month is my birthday?

Bonus Question: how many days would it take for me to collect 56,616 leaves?

Puzzle Copyright © Kevin Stone

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Hint

How many leaves will I have collected on day 5?

Answer

On the 23rd.

Reasoning

We could simply keep adding until we get the required number:

1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17 + 18 + 19 + 20 + 21 + 22 + 23

= 276 leaves.

But a more mathematical method might help to answer the Bonus Question – as this might take a while if we keep adding!

So, let's create a method by imagining that we are adding the numbers from 1 to 30.

1 + 2 + 3 + ... + 28 + 29 + 30

If we now take the numbers in pairs, taking one from each end, we have:

(1 + 30) + (2 + 29) + (3 + 28) + ... + (15 + 16)

Each pair adds to 31, and we have 15 pairs. So the total sum is 31 x 15 = 465.

The total sum from 1 to any number (N) can be found using this technique, and we will have:

Each pair adds to (1 + N), and there are N ÷ 2 pairs. So the total is:

(1 + N) x N

—

2

In this puzzle, we know that this equals 276.

So:

(1 + N) x N = 276

—

2

We can expand the brackets, and multiply both sides by 2, to give:

N + N^{2} = 552

Rearranging we get:

N^{2} + N - 552 = 0

And 552 = 2 x 2 x 2 x 3 x 23, so this can be factorised as:

(N + 24) x (N - 23) = 0

Because we need to find a positive number of days, the only possible answer is:

(N - 23) = 0

So N = 23 days.

Bonus Question

To answer the bonus question, we have:

(1 + N) x N = 56616

—

2

Rearranging we get:

N^{2} + N - 113232 = 0

And 113232 = 2^{4} x 3 x 7 x 337, so this can be factorised as:

(N - 336) x (N + 337) = 0

Because we need to find a positive number of days, the only possible answer is:

(N - 336) = 0

So N = 336 days (I did say that I liked collecting leaves!).