Discussion
A boy writes all the numbers from 100 to 999. The number of zeros he uses is \( a \), the number of 5s he uses is \( b \) and the number of 8s he uses is \( c \). What is the value of \( b + c - a \)?
Answer: B
Count each digit's appearances across the 900 three-digit numbers.
Zeros cannot occupy the hundreds place:
\[ \text{tens place} = 9 \times 10 = 90, \qquad \text{units place} = 9 \times 10 = 90 \]
\[ a = 180 \]
Fives may occupy any place:
\[ 100 \ (\text{hundreds}) + 90 + 90 = 280 \]
The same count applies to eights, so \( b = c = 280 \).
\[ b + c - a = 280 + 280 - 180 = 380 \]
Zero is the odd digit out because a three-digit number cannot begin with it.
No comment is present. Be the first to comment.
Loading…
Post your comment
Please log in or register to post a comment.
Examination Categories
- SSC Exams
- Banking Exams
- Teaching Exams
- Civil Services Exams
- Railways Exams
- Engineering Recruitment Exams
- Defence Exams
- State Government Exams
- Police Exams
- Insurance Exams
- Nursing Exams
- Other Government Exams
- NRA CET
- PG Entrance Exams
- Campus Placement
- MBA Entrance Exams
- CUET and UG Entrance
- Judiciary Exams
- Regulatory Body Exams
- Software Development