Solve the following riddle. I am thinking of a 4-digit number. The sum of the digits is 15. Starting from left, the third digit is double the first and the fourth is double the second. All the digits are different.
A) 2539
B) 1428
C) 1424
D) 2482
E) 1248

Question

Solve the following riddle. ​ I am thinking of a 4-digit number. The sum of the digits is 15. Starting from left, the third digit is double the first and the fourth is double the second. All the digits are different. ​
A) 2539
B) 1428
C) 1424
D) 2482
E) 1248

Quizplus · Accepted Answer

If we let the first digit be a, the second digit be b, the third digit be 2a, and the fourth digit be 2b, we have:
a + b + 2a + 2b = 15
3a + 3b = 15
a + b = 5
Since all the digits are different, we can only choose from the digits {1, 2, 3, 4}. We can quickly see that a = 1 and b = 4 is the only solution that satisfies both the sum of the digits and the condition that all digits are different. Therefore, the number is 1428.

Solve the Following Riddle