The mean age group of some students is 15years. When the age of a teacher, 45 years old, is added to the ages of the students, the mean of their ages become 18 years. Find the number of students in the group.
7
9
15
42
Explanation
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Total age of students=x Total number of students =y the starting average is therefore x/y=15 then age of teacher was added and the average became 18 that means x+45/y=18. To find y(total no of students) x+45/y is the same as x/y+45/y. x/y is 15 so 15+45/y=18 . Multiply all through with y. 15y+45=18y. Then (18y-15y=45) ,(3y=45) finally y =45/3 y=(15)
x/y=15,( x+45/y+1)=18 which makes it, x+45=18y+18, from the first equation x=15y, substitute x=15y into this equation (x+45=18y+18). therefore (15y+45=18y+18).(45-18=18y-15y). 27=3y divide both sides by 3 ans(y=9)
