Answer is 25 cm because the alphabets clearly visible from near place.
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i am asking F not U....
F meansĀ 
focal length
It should be 25 cm away from your eyes as it is the normal point of the eye.

if we try to read closer than 25 cm the words would be seen as blurred.
it happens due to power of accomodation of eye.
the minimum length of vision of eye is 25 cm
because eye cannot adjust its focal length below 25 cm.
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