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Ответ:

1) Put the beef, egg yolk, salt, pepper and oil into a large bowl.

2)Chop the onion and add it to all the other things in the bowl. Mix all this with your hands.

3)Make the mixture into four hamburger shapes. Press the flour into each side of each hamburger.

4)Heat the oil in a frying pan and fry your hamburgers.

5)Cut the bread rolls into two and toast each half.

6)Spread mustard and ketchup on each roll.