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Counting threes

By David, 9 January 2019 Brainteaser

Tilly started writing down numbers as she counted them out, “1, 2, 3, 4 …”
She was interrupted when she was writing down the 33rd 3.

What number was she in the middle of writing?

Scroll down for the answer!

 

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Brainteaser answer

Tilly would have been writing 138.

By the time Tilly gets to 100, she will have written twenty 3s: ten of them in the units (3, 13, 23 …) and ten of them in the tens (30, 31, 32 …). It will take three more 3s (103, 113, 123) before she needs to write any 3s in the tens. That leaves ten 3s to write, which you can easily count out.

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