For the most part, the cross-section formed is a <em>trapezoid</em>, but if the slice passes through the apex of the pyramid, that shape is a <em>triangle</em>.
Since she flipped the coin 20 times and got 4 heads and 6 tails, you put it as a fraction. So, it's 4/16 if you simplify 4 over 16 you get 1/4. You get this by dividing by 4 both numbers.