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We read the first line of the problem, then I added 2 sisters and 1 brother so that Sally had the same number of brothers and sisters. I saw that this wasn't right for Mark because he had 3 sisters and only 1 brother which wasn't twice as many. Then I added 1 more sister and 1 more brother. Sally had3 sisters and 3 brothers. Mark had4 sisters and 2 brothers. This meant that there were 7 children in the family.