Shortcut Trick
In every block of 10 consecutive numbers (e.g., 1-10, 11-20), there are exactly 2 numbers that have 1 or 7 in the unit's digit.
From 1 to 70, there are exactly 7 such blocks of 10 numbers.
Total favorable numbers = 7 blocks × 2 numbers per block = 14 numbers.
Required percentage = (14 ÷ 70) × 100% = 20%.
∴ The correct answer is 20%.
Alternate Method
Given: The range of numbers is from 1 to 70 (both included).
Total numbers (n) = 70
Formula Used: Percentage = (Favorable Numbers ÷ Total Numbers) × 100
⇒ Numbers with 1 in unit's digit: {1, 11, 21, 31, 41, 51, 61} → 7 numbers
⇒ Numbers with 7 in unit's digit: {7, 17, 27, 37, 47, 57, 67} → 7 numbers
⇒ Total favorable numbers = 7 + 7 = 14
⇒ Required Percentage = (14 ÷ 70) × 100
⇒ Percentage = (1 ÷ 5) × 100
⇒ Percentage = 20%
∴ The correct answer is 20%.
Additional Information
Unit Digit Distribution
In any sequence of 10 consecutive integers, each digit from 0 to 9 appears exactly once in the unit's place. This property is useful for quickly counting occurrences in large ranges.
Percentage to Fraction Conversion
To convert a percentage to a fraction, divide by 100. Common conversions include 20% = 1/5, 25% = 1/4, 40% = 2/5, and 50% = 1/2.
Total Elements in a Range
For a range from 'a' to 'b' (both inclusive), the total number of terms is given by the formula: (b − a) + 1.