Two lenses are in contact. One of the lenses has a focal length of +10.0cm when used alone. When the two are in combination, an object 20cm away from the lenses forms a real image 40cm away from the lenses. What is the focal length?

I am not sure how to approach this problem. I will like to see detailed steps on how to do this problem.
I tried using the formula 1/fe= 1/f1 + 1/f2

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  1. 1/f=1/di+1/do

    let do be at the original 20cm. Then those rays enter the second lens, and forms an image at 40cm

    f= 40/3 cm

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