How many integers are there between 100 and 1000 all of whose digits are even? A. 60 B. 80 C. 100 D. 90

GATE 2019 · General Aptitude · Arithmetic Series · medium

Answer: 100 integers between 100 and 1000 have all even digits.

1. Identify even digits: All digits must come from this set of 5 values.
2. Count choices for hundreds digit: We exclude 0 from the hundreds position. So 4 valid choices.
3. Count choices for tens and units digits: Both remaining positions can freely use any of the 5 even digits.
4. Apply multiplication principle: There are 100 three-digit integers with all even digits.