Question 758125
the definition of average is:
( total value of {{{ n }}} items ) / {{{n }}} = average value of {{{ n }}} items
given:
( sum of ages ) / 30 = 12
Sum of ages = 360 
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5 of the 30 students average 10 yrs old
( sum of ages of this group ) / 5 = 10
sum of ages of this group =  50 
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another group of 5 of the 30 students 
average 14 yrs old
( sum of ages of this group ) / 5 = 14
sum of ages of this group =  70
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There are 30 - ( 5 + 5 ) = 20
students remaining
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For the remaining students:
{{{ ( 360 - ( 50 + 70 ) ) / 20 }}}
{{{ ( 360 - 120 ) / 20 }}}
{{{ 240 / 20 = 12 }}}
The average age of remaining students is 12 yrs old