Shortcut Trick
Divisibility of 425139z2 by 9 ⇒ Sum of digits (26 + z) is divisible by 9 ⇒ z = 1.
Divisibility of 3x5479y4 by 8 ⇒ Last 3 digits (9y4) are divisible by 8 ⇒ y can be 0, 4, or 8.
Divisibility by 11 ⇒ Difference of odd/even position sums: (x + 17) − (y + 15) = x − y + 2 must be 0 or 11.
Testing y values gives valid (x, y) pairs: (9, 0), (2, 4), and (6, 8).
Evaluate 3x + 2y − z for each: 26, 13, and 33. The maximum value is 33.
∴ The correct answer is 33.
Alternate Method
Given:
8-digit number 3x5479y4 is divisible by 88.
8-digit number 425139z2 is divisible by 9.
Concept Used:
Divisibility by 88 implies the number is divisible by both 8 and 11.
Divisibility by 8: The last 3 digits of the number must be divisible by 8.
Divisibility by 11: The difference between the sum of digits at odd places and the sum of digits at even places must be 0 or a multiple of 11.
Divisibility by 9: The sum of all digits must be divisible by 9.
Calculations:
Step 1: Find z using the divisibility rule of 9 for 425139z2.
⇒ Sum of digits = 4 + 2 + 5 + 1 + 3 + 9 + z + 2 = 26 + z
⇒ The nearest multiple of 9 greater than 26 is 27.
⇒ 26 + z = 27
⇒ z = 1
Step 2: Find y using the divisibility rule of 8 for 3x5479y4.
⇒ The last 3 digits 9y4 must be divisible by 8.
⇒ If y = 0, 904 ÷ 8 = 113 (Divisible)
⇒ If y = 4, 944 ÷ 8 = 118 (Divisible)
⇒ If y = 8, 984 ÷ 8 = 123 (Divisible)
⇒ Possible values for y are 0, 4, and 8.
Step 3: Find x using the divisibility rule of 11.
⇒ Sum of digits at odd places (from the left) = 3 + 5 + 7 + y = y + 15
⇒ Sum of digits at even places (from the left) = x + 4 + 9 + 4 = x + 17
⇒ Difference = (x + 17) − (y + 15) = x − y + 2
⇒ For divisibility by 11, (x − y + 2) must be 0 or 11.
Checking possible pairs for (x, y):
⇒ Case 1: If y = 0, x − 0 + 2 = 11 ⇒ x = 9
⇒ Case 2: If y = 4, x − 4 + 2 = 0 ⇒ x = 2
⇒ Case 3: If y = 8, x − 8 + 2 = 0 ⇒ x = 6
Step 4: Find the greatest possible value of (3x + 2y − z).
⇒ For x = 9, y = 0, z = 1 ⇒ 3(9) + 2(0) − 1 = 27 + 0 − 1 = 26
⇒ For x = 2, y = 4, z = 1 ⇒ 3(2) + 2(4) − 1 = 6 + 8 − 1 = 13
⇒ For x = 6, y = 8, z = 1 ⇒ 3(6) + 2(8) − 1 = 18 + 16 − 1 = 33
⇒ The greatest possible value obtained is 33.
∴ The correct answer is 33.
Additional Information
Divisibility Rule of 8
A number is divisible by 8 if the number formed by its last three digits is a multiple of 8.
Divisibility Rule of 9
A number is divisible by 9 if the total sum of all its digits is completely divisible by 9.
Divisibility Rule of 11
A number is divisible by 11 if the difference between the sum of digits at odd positions and even positions is either 0 or a multiple of 11.
Divisibility of Composite Numbers
To check divisibility for a composite number like 88, simply verify divisibility for its co-prime factors, which are 8 and 11.