Question

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

Answer 1

8

Explanation:

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 x 2 x 2 = 8 integers.

(Here are the numbers listed out):

222

225

252

255

522

525

552

555

Answer 2

8

Explanation:

There are two choices for each digit, and 3 digits, so 2^3 = 8 possible numbers. Here's a list:

222, 225, 252, 255

522, 525, 552, 555