Can someone please help me with this ONE algebra question ?

Question

Find the focal length of the magnifying glass if you can see the butterfly clearly when your eyes is 5.5 cm away from the glass and the glass is 25 cm away from the butterfly. The Formula is 1/s + 1/t =1/f.

A.) focal length is about 30.5 cm
B) focal length is about 4.5 cm
C.)Focal length is about 4.2 cm

Answer 1

Distance between butterfly and the glass (s) = 5.5

Distance between eyes and the glass (t) = 25cm

The focal length of the magnifying glass formula : 1 / f = 1 / s + 1 / t

f = focal length of the glass

Substitute s = 11 / 2cm and t = 25cm in the above formula

1 / f = 1 / (11 / 2) + 1 / 25

1 / f = 2 / 11 + 1 / 25

1 / f =  50 + 11 / 25×11

1 / f = 61 / 275

f = 275 / 61

f = 4.5cm (Approximately)

B is the right choice.