For each number, there are four digits, and for each digit, there are two possibilities for digits: 3 or 4. So the number of 4-digit numbers is 2*2*2*2 = 16. The 4-digit numbers using 3 and 4 are:
3333
3334
3343
3344
3433
3434
3443
3444
4333
4334
4343
4344
4433
4434
4443
4444
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three.. 2, 22, 222
24.
120
You would need infinitely many digits to write all numbers. However, to write all whole number (integers) you would need 4243.
13