Four different digits from 1 to 9 are required to open a safe. 1. The sum of the digits is 20.
2. The first digit is greater than the third.
3. The second and fourth digits differ by at least 5.
4. Exactly two digits are squares.
5. The first and fourth digits add up to a prime number.
6. The fourth digit is the lowest.
Can you find the four-digit combination?

 5942

Step-by-step explanation:

Clue 4 states exactly two of the digits = 1, 4, or 9

Clue 1 leaves us with the following combinations:

1, 9, 2, 8

1, 9, 3, 7    eliminate by clue 5

4, 9, 2, 5

1, 4, 7, 8

Clue 5 directs us to the following order for 1,9,2,8

2 __ __ 1     --> 2981 or 2891   eliminate by clue 2

9 __ __ 8    --> 9128 or 9218   eliminate by clue 6

9 __ __ 2    --> 9182 or 9812   eliminate by clue 6

Clue 5 directs us to the following order for 4,9,2,5

5 __ __ 2     --> 5492 or 5942   eliminate 5492 by clue 2

9 __ __ 2    --> 9452 or 9542   eliminate by clue 3

Clue 5 directs us to the following order for 1,4,7,8

4 __ __ 1    --> 4781 or 4871    eliminate by clue 2

The only combination not eliminated is 5-9-4-2, which satisfies all six clues.

1) 5 + 9 + 4 + 2 = 20

2) 5 > 4

3) 9 - 2 > 5

4) 4 & 9 but not 1 are included

5) 5 + 2 = 7, which is a prime number

6) 2 <  5, 9, 4