When an object is placed 60 cm from a converging lens, it froms a real image. When the object is moved to 40?

cm from the lens, the image moves 10 cm farther from the lens. What is the focal length of the lens? Give your answer in cm with one decimal point accuracy.When an object is placed 60 cm from a converging lens, it froms a real image. When the object is moved to 40?
the lens equation is 1/F=1/Dobject+1/Dimage


for the first part 1/F=1/60+1/Dimage


then for the 2nd part the image is now Dimage +10 and Dobject is 40 giving 1/F=1/40+1/(Dimage +10)


set the equations equal 1/60+1/Dimage=1/40+1/(Dimage+10)


solve for Dimage and you get Dimage=-40 or 30


however the image is real so the positive one is used


F=30cm