A parallel beam of monochromatic light falls on a combination of a convex lens and a concave lens of focal length 15 centimetre and 5 centimetre respectively .what is the distance between the two lenses to obtain a parallel beam of light from the concave lens?

Use formula for two lens system here,  the effective Focal length of such system is
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Why there is -1/5 can u plzz tell!
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Because 5 cm is focal length of concave lens
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Because it's of concave lens
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10 cm
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formula used is 
1/F = 1/f1 +1/f- d/ f1f2
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Focal length of concave lens is negative that's why
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