Question 721137
For {{{n}}} scores {{{average=sum/n}}}
For the {{{20}}} girls the sum of the scores was {{{sum[g]}}} and
{{{85=sum[g]/20}}} --> {{{sum[g]=85*20=1700}}}
For the class, the sum of the {{{30}}} scores is {{{sum[c]}}},
and the average is {{{80}}}={{{sum[c]/30}}} --> {{{sum[c]=80*30=2400}}}
Since {{{sum[c]}}} is the sum of the boys' scores, {{{sum[b]}}} ,
plus the sum of the girls' scores
{{{sum[b]+1700=2400}}} --> {{{sum[b]=2400-1700}}} --> {{{sum[b]=700}}}
The average for the boys is the sum of their scores divided by the number of boys:
{{{700/10=highlight(70)}}}