you could split the boys in half by 2 groups of 7 then use one of those seven group and add it to the 21 girls which is 28. Then divide 28 by 7 and you get 4. So there is 5 with 7 members in each. A more easy way of doing it is adding 14 and 21 together and you get 35 and 7x5 is 35. You can also do 7 groups of 5.