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help

0
171
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+626

How many three-digit positive integers exist, all of whose digits are 2's and/or 5's?

Oct 9, 2018

#1
+99618
+1

222

225

252

255

522

525

552

555

Oct 9, 2018
#2
+2340
0

Listing all the possible arrangements is one way to approach this problem; however, if I told you that this number has 10  or 100 digits, would you be as willing to write out all of them? I doubt it.

The first digit has two possibilities; it can either be a 2 or a 5.

The second digit has two possibilities; it can either be a 2 or a 5.

The third digit has two possibilities; it can either be a 2 or a 5.

$$2*2*2=8 \text{ integers}$$

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Oct 9, 2018