Shortcut Trick
By the divisibility rule of 11, the difference between the sums of alternating digits must be 0 or a multiple of 11.
Sum of odd-placed digits = 7 + 4 + K + 5 = 16 + K
Sum of even-placed digits = 8 + 5 + 8 + 4 = 25
Equating the difference to 0: 25 − (16 + K) = 0 ⇒ 9 − K = 0 ⇒ K = 9
∴ The correct answer is 9.
Alternate Method
Given:
The number 7845K854 is perfectly divisible by 11.
Formula Used:
A number is divisible by 11 if the difference between the sum of its digits in odd positions and the sum of its digits in even positions is either 0 or a multiple of 11.
Calculations:
⇒ Let's find the sum of the digits at the odd positions (1st, 3rd, 5th, and 7th from left):
⇒ S1 = 7 + 4 + K + 5 = 16 + K
⇒ Let's find the sum of the digits at the even positions (2nd, 4th, 6th, and 8th from left):
⇒ S2 = 8 + 5 + 8 + 4 = 25
⇒ Find the difference between the two alternating sums:
⇒ Difference = S2 − S1 = 25 − (16 + K) = 9 − K
⇒ For the whole number to be divisible by 11, the difference must be 0, 11, 22, etc.
⇒ Since K is a single-digit positive integer (from 0 to 9), the only possible valid difference is 0:
⇒ 9 − K = 0
⇒ K = 9
∴ The correct answer is 9.
Additional Information
Divisibility Rule of 3 & 9
A number is divisible by 3 or 9 if the sum of all its digits is completely divisible by 3 or 9, respectively.
Divisibility Rule of 4 & 8
A number is divisible by 4 if the last two digits form a number divisible by 4. It is divisible by 8 if the last three digits form a number divisible by 8.
Divisibility Rule of 7, 11 & 13 (Combined Block Trick)
Group the number into blocks of 3 digits moving from right to left. If the alternating difference of these blocks is divisible by 7, 11, or 13, the original number is divisible by the respective divisor.