Question 503991
The average is 
( total of all scores) / ( number of tested students)
given:
Number of tested students = {{{50}}}
Total of scores by juniors = {{{ 35*80 = 2800 }}}
Total of scores by seniors = {{{ 15*70 = 1050 }}}
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{{{ ( 2800 + 1050 ) / 50 = 3850/50 }}}
{{{ 3850/50 = 77 }}}
Average of all 50 students = 77
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Notice that the average in NOT the average of
the 2 averages, which would be {{{ ( 80 + 70 )/2 = 75 }}}