Well depending on how you look at you could do $40 plus that extra 20% and since you said that means to add you would do the same price. as the actual price before you add that thirty 20% so you would add 40 plus 40 equals 80. If you did 25% the you would add 40 and 40 which is 80 and that's 20%out of 25% and add 5 to 80 and you get $85.
I only know D. but I'll try to figure out the rest too. D. 9 years old
Check the picture below.
keeping in mind that the point of tangency for a radius line and a tangent is alway a right-angle, since the "red" chord is parallel to the "green" tangent line outside, then the chord is cutting the "green" radius there in two equal halves at a right-angle, as you see in the picture.
we know the chord is 10 units long, so 5 + 5, since is perpendicularity with the radius will also cut the chord in two equal halves.
anyhow, all that said, we end up with triangle you see on the right-hand-side, and then we can just use the pythagorean theorem.