Jim was out in the forest and found a long narrow lake.he wanted to estimate the length of the lake, but he did not have anything to measure with.he walked at

Use Pythagoras theorem:

(65 x 1.5)^2 - (25 x 1.5)^2=L^2

L^2=8,100, take the square root of both sides,

L=90 Feet, or 90/1.5=60 paces, the length of the lake.

Hi wondergirl,

Did you get a picture with this?  The words are not self explanatory.    

When he first stumbled on this long skinny lake, was he at the top of the skinny end or was he somewhere along a long side or what?

Yes guest assuming that when jim stumbles on the lake he is at the top of the skinny end then what you have done is perfect.