Ira bought a t-shirt that was discounted 50%. He used a coupon for 50% off at the cash register. Why was he charged $6.50 instead of $0?

50% off makes the new price half of the original, then another 50 % would be half off the discounted price, so they actually end up paying 25 % of the original price:

50 % x 50 % = 25 %

i'm not sure i quite understand, could you provide more information?

b.-4 units

all you   do is count how many times you move