Imagine a square ABCD with a diagonal BD. Now draw a line EF parallel to BD, such that E lies on BC and F lies on CD. Also length of EF = length of AB. Now Colour the space enclosed by BDFE.
Of the square ABCD, what percentage of area is coloured ?
I get a ratio of 3/8 of the whole area, 37.5%
EF=AB, so CF=EC=1/2*AB
Then just find the area of Triangle CFE and subtract it from Triangle
BCD. and then divide the answer by the total area of square ABCD.