Answer :

konrad509
[tex]7x=21\\ x=3\\\\ 4x=4\cdot3=\boxed{12}[/tex]
[tex]boys:girls=4:7\\\\girls=21\\\\therefore\\\\boys:21=4:7\\\\\frac{boys}{21}=\frac{4}{7}\ \ \ \ \ |cross\ myltiply\\\\7\ boys=4\times21\\\\7boys=84\ \ \ \ \ |divide\ both\ sides\ by\ 7\\\\boys=84:7\\\\\boxed{boys=12}[/tex]

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