Question

the average age of 30 students of a class is 13 years out of them the average of 18 students is 15 years find the average age of the remaining students

Answer 1

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Your answer
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Average age of 30 students in a class = 13 years

Sum of their ages = (30 × 13 ) years = 390 years

Out of them, average age of 18 students = 15 years

Sum of the ages of 18 students = (18 × 15) years = 270 years

Then,
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Remaning number of students = (30 - 18) = 12

Sum of the ages of 12 students = 390 - 270 = 120 years

Then,
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Average age of remaining 12 students = 120/12 years = 10 years

HOPE IT HELPS

Answer 2

                         Sum of Observations
AVERAGE   =   ------------------------------
                         Number of observation

Sum of the ages of 30 students = 30 * 13 =390

(Addition of  the ages of all the 30 students sums to 390 years)


Sum of the ages of 18 students = 18*15=270 years

(Addition of  the ages of 18 students sums to 270 years)


Remaining sum of the ages of 12 students will be =390-270=120

Hence the average will be

Sum of the observations=120
Number of observations=10

                     120 
Average =   ---------      = 10 years =)ANSWER 
                      10