physics help please!!?

Di/Do = Hi/Ho = magnification  (Ho and Hi are the heights of the object and image respectively.)

hence Di/Do = 0.7
so Di = 0.7  Do

But Di = 30 cm (given in problem)


Now 1/Do + 1/Di = 1/f
1/f = 1/Do +1/(0.7Do)
    = (0.7 +1)/(0.7 Do) = 1.7/(0.7Do)   (using normal addition of fraction rules.)

so f = Do * 0.7/1.7
= 30 * 0.7 * 0.7 / 1.7  
= 8.86 cm