Question

How many 2-digit numbers can be formed using the digits 1 and 7?

Answer 1

Step 1: When we are not repeating the digits:
Fix 1 at the tens place. ​

• Fixing 1 in the tens place we get 17.

When we fix 7 at the tens place

• Fixing 7 in the tens place we get, 71.

So we get 2 numbers 17 and 71.

Step 2: When we are repeating the digits
Fix 1 at the tens place. ​

• Fixing 1 in the tens place we get, 11 and 17.

When we fix 7 at the tens place

• Fixing 7 in the tens place we get, 71 and 77>

So we are getting 4 numbers in total: 11, 17, 71 and 77.