notice one of the trapezoids in the picture below
the slanted part is 15.8, parallel bases are 10 and 15
and the vertical part is 15 also
now... [tex]\bf \textit{area of a trapezoid}=A=\cfrac{h}{2}(base1+base2)[/tex]
where,
h=height
base1 = one of the parallel sides
base2 = the other parallel side
