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  4. A clock hung on a wall has marks instead of numerals on its dial on the adjoining wall there is a plane mirror and the image of the clock in the mirror indicates the time 7: 10. Then the time on the clock is:

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Q. A clock hung on a wall has marks instead of numerals on its dial on the adjoining wall there is a plane mirror and the image of the clock in the mirror indicates the time $7 : 10$. Then the time on the clock is:

BHUBHU 2005

Solution:

In a plane mirror the image undegoes depth inversion hence, the time on the image is as much ahead of $12\, h\, 0\, \min$ as the time on actual clock is behind $12\, h\, 0 \,\min$.
Time on the actual clock $=7 \,h\, 10\, m$.
Time on the image clock
$=(12 \,h$ and $0 \,\min )-(7\, h$ and $10\, \min )$
$\therefore 4\, h$ and $50\,\min$