Question 1194430
Draw the described triangle and label the parts.


Width of the lake,  {{{760/tan(45)+760/tan(20)}}}



In fact, think about the side with the 45 degree angle of depression.  If you drew two connected right triangles then one length part along the bottom is 760 m, just as the height of the point above the lake.  

Expression for lake width can then be  {{{760+760/tan(20)}}}.