Enrico was 0.2 miles from the beach when he realized he didn't have his keys.
So if Enrico didn't forget his keys he would've only had to walk 0.2 miles to the beach.
But instead he had to 0.2 mi back to home, and 0.4 mi back to the beach. He had to walk 0.6 miles.
So subtract the amount he had to walk to get his keys (0.6) by the distance he could've walked if he had them (0.2).
He walked an extra 0.4 miles for his mistake.