maths
posted by SABRINA .
please help
A circular opaque disk, 25 cm in diameter, is set up 30 cm from a circular source of light of diameter 40 cm. A screen is placed 30 cm beyond the opaque disk. Describe, giving dimensions, the nature of the shadow on the screen

maths 
SABRINA
i have worked out the diameter of the umbra to be 25 cm, is this correct?....
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