Step-by-step explanation:
the additional 10% off are based on the already discounted 50% of the original price (50% off means also 50% remain).
so, 10% off of 50% of the original price is then 5% off of the original price (half of the discount the 10% would have been based on the original 100%).
so, he received 50% + 5% = 55% discount from the original price.