How is finding the area of a parallelogram like finding the area of two congruent triangles?

If you look at either diagonal (line joining opposite angles) it forms 2 triangles that are identical, both in size and shape, but that are in different positions. So by finding the area of one using base x height ÷ 2, and then adding them together, you get the area of the polygon.
However, you will find that that would be the same as just doing base x height, so the area of a parallelogram is just that.

$250 deductible means jane must pay the first $250 of her bill. so the insurance company paid = (80% of bill) - deductible = (0.8 x $2550) - $250 = $2040 - $250 = $1790 and jane's cost is = (bill) - (insurance payment) = $2550 - $1790 = $760