Tell him to treat boys as four and the whole class as seven. Then "7-4 = 3" is the number of girls. Then the number of girls accounts for 3/7 of the class. Girls account for 3/4 of boys. That is, the class size is "1", 1-4/7=3/7.

You can use a pencil instead. Three represents girls. Four represents boys.

Is there such a topic in grade three? We only met in the fifth grade, alas _ _ _ _ _ _ _.