At Teachem High, $$20 \%$$ of the pupils normally at school are in Year $$7$$.  One day some Year $$7$$ girls miss school for a sports match. The total number of pupils left at school is then $$400$$ and the proportion of these pupils who are in Year $$7$$ is $$10 \%$$.  How many pupils are normally at school in total?  Answer:~\uline{~~~~~~~~~~}~ 
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