At what time between 6 π.π. and 7 π.π. will the minute hand and hour handβ¦
2014
At what time betweenΒ 6Β π.π. andΒ 7Β π.π. will the minute hand and hour hand of a clock make an angle closest toΒ 60β?
- A.
6: 22 π.π.
- B.
6: 27 π. π.
- C.
6: 38 π.π.
- D.
6: 45 π.Β π.
Attempted by 165 students.
Show answer & explanation
Correct answer: A
Key idea: If t is minutes after 6:00, the minute hand is at 6t degrees and the hour hand is at 180 + 0.5t degrees. The angle between them is |6t β (180 + 0.5t)| = |5.5t β 180|.
Solve |5.5t β 180| = 60. This gives two linear equations.
5.5t β 180 = 60 β 5.5t = 240 β t = 240/5.5 = 480/11 β 43.636 minutes β about 6:43:38.
5.5t β 180 = β60 β 5.5t = 120 β t = 120/5.5 = 240/11 β 21.818 minutes β about 6:21:49.
Check which given time is closest to either exact solution by computing the angle at each provided time:
At 6:22 (t = 22): angle = |5.5Β·22 β 180| = |121 β 180| = 59Β° (1Β° away from 60Β°).
At 6:27 (t = 27): angle = |5.5Β·27 β 180| = |148.5 β 180| = 31.5Β°.
At 6:38 (t = 38): angle = |5.5Β·38 β 180| = |209 β 180| = 29Β°.
At 6:45 (t = 45): angle = |5.5Β·45 β 180| = |247.5 β 180| = 67.5Β° (7.5Β° away from 60Β°).
Conclusion: The times when the hands make exactly 60Β° between 6 and 7 are about 6:21.82 and 6:43.64. Among the given choices, 6:22 is the closest.
Answer: 6:22 a.m.