How many 3-digit numbers can you write using only digits 1 and 7? (Of course, digits can repeat.) How many 3-digit numbers can you write using only digits 1 and 7? (Of course, digits can repeat.)

Short answer 2 * 2 * 2 = 8

Step-by-step explanation:

The first digit can be either a 1 or a 7. You have 2 choices.

Once you have decided you can write either a 1 or 7 for the second which is 2 choices

Same with the third.

The numbers are

1 * 1 * 1

1 * 1 * 7

1 * 7 * 1

1 * 7 * 7

7 * 1 * 1

7 * 1 * 7

7 * 7 * 1

7 * 7 * 7

111,117,177,777,771,711,717,171