Răspuns:
voi nota nr elevilor cu a
3a/10 din elevi sunt fete de 12 ani
a-3a/10=10a/10-3a/10=7a/10 (restul)
1/3×7a/10=7a/30 (baieti de 12 ani)
3a/10+7a/30+14=a
9a/30+7a/30+420/30=30a/30
16a/30+420/30=30a/30
420/30=30a/30-16a/30
420/30=14a/30
a=420:14
a=30 elevi