The distance across the lake is 3.5 km
Hypotenuse is divided in equal parts. Thus both sides are divided in the same proportion.
find the length of the bottom
7•X( full length) and y•(x/2)
use scale factors (ratios)
7:y so 7/y=scale factor
(full length):(haft the length)
(full lengtb)/(haft the length)
( make sure you know in to km) (ruler)
find area of triangle 1/2bh
the one problem would be of cource the missing area of the shaded part.
but what im guessing is you might need to find where each end of the shaded region ends (measure the farthest distances of shaded region) and start subtracting area of the triangle. (parts of the triangle that you know have no shaded parts to them).
There is also another way but you have to cut the paper and find total burn time for that shaded part and the triangle and than convert to km
find the distance across the lake