Question 814690
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Hi,
Average age of students of an adult school is 40 years.
Let x represent the total number of these students,  
40x would be their Total Ages.
120 new students whose average age is 32 years joined the school
{{{ 120*32 = 3840}}} would be their Total Ages
As a result the average age is decreased by 4 years.
Question states***
{{{ (40x + 3840)/(x + 120) = 36}}}
    40x + 3840 = 36(x+120)
     4x = 480
      x = 120 students originally, Now 240 students.
CHECKING our Answer***
{{{(120*40 + 120*32)/240 = 8640/240 = 36 }}}