Question

2 lenses of power +10D. and -5d

Answer 1

1/6 + 1/x = 1/10
1/x = 1/10 - 1/6
1/x = (3-5)/30 = -2/30 
= -1/15
x = -15 
1/f = -15
f = -1/15 = -0.0666 m 
f of lens having power +6D = 1/6 = +0.1666 m