The focal length of the combination is given by 1/ f combination (1/fc ) = 1/f + 1/Fm
Assuming the the near point is 30 cm
1/f = 1/30 + 1/v
At 15 cm
1/fc = 1/15 + 1/v
1/fc = 1/15 + 1/f – 1/30
1/fc – 1/f = 1/30
1/f + 1/Fm – 1/f = 1/30
1/Fm = 1/30
Fm = 30 cm
Hope this answers. Have a nice day.
