Brain Teasers
Minefield
A road leading to a secret military base is 20 feet wide. Private Roberts was ordered to plant 20 land mines on the road in a 20-foot by 20-foot area. He laid out the area in a grid, with 100 2-foot squares arranged in 10 columns (running parallel with the road) and 10 rows (running perpendicular with the road). He planted each land mine within a square on the grid so that there were exactly two mines in each row and two in each column. When he was finished, he found himself on the wrong side of the minefield, and he couldn't get back to the base without crossing the minefield.
He labeled the columns A-J from left to right, and numbered the rows 1-10 starting with the row closest to the base.
-- A B C D E F G H I J
01X X X X X X X X X X
02X X X X X X X X X X
03X X X X X X X X X X
04X X X X X X X X X X
05X X X X X X X X X X
06X X X X X X X X X X
07X X X X X X X X X X
08X X X X X X X X X X
09X X X X X X X X X X
10X X X X X X X X X X
He could remember the following facts about where the mines were planted:
1. There was no mine in any of the four corners.
2. Row 10 was the only row where the two mines were next to each other.
3. The two mines in column C were in the same rows as the two mines in column H.
4. One of the mines in column B was in row 7.
5. In row 9, there were 3 empty squares between the two mines.
6. Both mines in column E were in row 5 or higher.
7. One of the mines in column I was in row 9 or 10.
8. Both mines in column H were in odd number rows.
9. The two mines in column D were in the same rows as the two mines in column J.
10. One of the mines in column A was in row 5.
Can you find a safe path to the other side of the mine field moving one square at a time (either horizontally, vertically, or diagonally), starting in row 10 and moving through row one?
He labeled the columns A-J from left to right, and numbered the rows 1-10 starting with the row closest to the base.
-- A B C D E F G H I J
01X X X X X X X X X X
02X X X X X X X X X X
03X X X X X X X X X X
04X X X X X X X X X X
05X X X X X X X X X X
06X X X X X X X X X X
07X X X X X X X X X X
08X X X X X X X X X X
09X X X X X X X X X X
10X X X X X X X X X X
He could remember the following facts about where the mines were planted:
1. There was no mine in any of the four corners.
2. Row 10 was the only row where the two mines were next to each other.
3. The two mines in column C were in the same rows as the two mines in column H.
4. One of the mines in column B was in row 7.
5. In row 9, there were 3 empty squares between the two mines.
6. Both mines in column E were in row 5 or higher.
7. One of the mines in column I was in row 9 or 10.
8. Both mines in column H were in odd number rows.
9. The two mines in column D were in the same rows as the two mines in column J.
10. One of the mines in column A was in row 5.
Can you find a safe path to the other side of the mine field moving one square at a time (either horizontally, vertically, or diagonally), starting in row 10 and moving through row one?
Hint
Try to map the safe squares, not the mines. Diagonal movements are allowed.Answer
The shortest safe path: start at C:10 and walk straight to C:5, turn right to D:5, then move diagonal to E:4, and walk straight out through E:1. Following is a map of the safe squares.-- A B C D E F G H I J
01S X X S S X X X X S
02X X S X S X X S X X
03X X X X S X X X X X
04X X S X S X X S X X
05X S S S X X X S X S
06X X S X X X X S X X
07S X S S X X X S X S
08X X S X X X X S X X
09S S S S X S S S X S
10S X S X X X X S X S
Row 10: A, C, H and J are safe (clue 1, 3, 8).
Row 9: Clue 1, 2, 5, 7 place mines in I:9 and E:9.
Row 8: C and H are safe (clue 3 and 8).
Row 7: There is a mine in B (clue 4), which makes A,C,D, H, and J safe (clue 2,3, and 9).
Row 6: C and H are safe (clue 3 and 8).
Row 5: There is a mine in A (clue 10), which makes B,C,D, H, and J safe (clue 2,3, and 9).
Row 1 through 4: E is safe (clue 6).
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