F.2 Maths 一題!

有張圖:
http://img24.imageshack.us/img24/1503/gggxs.png

and..問題:)

A mosquito flies from a point B on water surface along the straight line BD to a point D (see the figure). It is caught by a dragonfly flying from a point C on the water surface along the straight line CD. If BD and CD make angles of 70° and 25° respectively with the water surface AC and the distance between B and C is 80cm, find the height(AD) of D above the water surface.
(Give the answer correct to the nearest cm.)

唔該晒:)