What power lens should he use as a magnifier to see clearly at a distance of 15 cm without wearing his glasses?

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.

2hydrogen and 1 covalent