Why do this problem?
requires logical thinking and could be a good way to introduce a trial and improvement approach. Children need to understand the difference between having a certain number of brothers and the number of boys in a family.
You could start with a discussion on how many brothers and sisters the members of the class have and relate this to the number of boys and girls in their families.
Children could then work in pairs on the problem either using paper/miniwhiteboards to make jottings. It would be worth stopping them after a few minutes to see how they are going about tackling the problem and to share some ideas. You could also draw attention to different ways of representing the problem that you have noticed.
You may wish to introduce the idea of recording in a table. Some children could be challenged to design their own table while you could give others this sheet to complete. Filling in the first line or two of the table together as a class would give them a
How many brothers has Sally got now?
How many brothers has Mark got now?
How many boys are there in the family now?
Children could make up different problems for each other such as "What would the answer be if Mark had three times as many sisters as brothers?" Alternatively, they could make up similar problems about their own, or invented, families.
Some learners would benefit from using something to represent the boys and girls, for example counters in two colours.