How many times will you get lucky?

Probability Level 2

How many times does the number " $7$ " show in the list of numbers from $1$ to $1000$ ?

197

300

200

100

1 solution

Jeremy Bansil
Aug 22, 2014

If you would like to go on the hard way, go list all numbers from 1 to 1000 on computer, and press "Ctrl+F" and count all 7's.

If you would like to have an easy way... Well, there are a lot.

could you mention the easy wasys !!!!

manoj ram - 6 years, 8 months ago

So here is the technique: there are a hundred $7$ 's on all units digit, a hundred $7$ 's on all tens digit, and a hundred $7$ 's on all hundreds digit. And it sums up to $300$

Jeremy Bansil - 6 years, 7 months ago

Najlepiej myśleć tu jak dziecko i doliczymy sie 300 siódemek.(300 x 7)

Irena Neumann - 6 years, 4 months ago

My solution for this was like this. For 1~99 it has num7 in each 10 numbers, while there is 9 more in 70,70,72.. till 79 without repeating 77. So for every 100 numbers, it has 19 numb 7. So for 1000, we multiple 19 x 10 = 190. For number 700~799, every number of it has the num 7 so we need to add more which the complete calculation will be like: 19x10+(100-19)=271

Jingchong Jc - 6 years, 8 months ago

How many times will you get lucky?

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