Ex.1:Find the average of all
prime numbers between 30 and 50?
Sol: there are five prime
numbers between 30 and 50.
They are 31,37,41,43 and 47.
Therefore
the required average=(31+37+41+43+47)/5 ó199/5 ó 39.8.
Ex.2. find the average of
first 40 natural numbers?
Sol: sum
of first n natural numbers=n(n+1)/2;
So,sum
of 40 natural numbers=(40*41)/2 ó820.
Therefore the required average=(820/40) ó20.5.
Ex.3. find the average of
first 20 multiples of 7?
Sol: Required average =7(1+2+3+…….+20)/20 ó(7*20*21)/(20*2) ó(147/2)=73.5.
Ex.4. the average of four
consecutive even numbers is 27. find the largest of these numbers?
Sol: let the numbers be x,x+2,x+4 andx+6. then,
(x+(x+2)+(x+4)+(x+6))/4) =
27
ó(4x+12)/4 = 27
óx+3=27 ó
x=24.
Therefore the largest
number=(x+6)=24+6=30.
Ex.5. there are two sections A
and B of a class consisting of 36 and 44 students respectively. If the average
weight of section A is 40kg and that of section B is 35kg, find the average
weight of the whole class?
Sol: total weight of(36+44)
students=(36*40+44*35)kg =2980kg.
Therefore
weight of the total class=(2980/80)kg
=37.25kg.
Ex:6.nine persons went to a
hotel for taking their meals 8 of them spent Rs.12 each on their meals and the
ninth spent Rs.8 more than the average expenditure of all the nine.What was the
total money spent by them?
Sol: Let the average
expenditure of all nine be Rs.x
Then 12*8+(x+8)=9x or 8x=104 or
x=13.
Total money spent =
9x=Rs.(9*13)=Rs.117.
Ex.7: Of the three numbers, second
is twice the first and is also thrice the third.If the average of the three
numbers is 44.Find the largest number.
Sol: Let the third number
be x.
Then second number = 3x.
First number=3x/2.
Therefore x+3x+(3x/2)=(44*3) or
x=24
So largest number= 2nd
number=3x=72.
Ex.8:The average of 25 result is 18.The average of 1st
12 of them is 14 & that of last 12 is 17.Find the 13th result.
Sol: Clearly 13th
result=(sum of 25 results)-(sum of 24 results)
=(18*25)-(14*12)+(17*12)
=450-(168+204)
=450-372
=78.
Ex.9:The Average of 11 results
is 16, if the average of the 1st 6 results is 58 & that of the
last 63. Find the 6th result.
Sol: 6th result
= (58*6+63*6-60*11)=66
Ex.10:The average waight of
A,B,C is 45 Kg. The avg wgt of A & B be 40Kg & that of B,C be 43Kg.
Find the wgt of B.
Sol. Let A,B,c represent
their individual wgts.
Then,
A+B+C=(45*3)Kg=135Kg
A+B=(40*2)Kg=80Kg &
B+C=(43*2)Kg=86Kg
B=(A+B)+(B+C)-(A+B+C)
=(80+86-135)Kg
=31Kg.
Ex. 11. The average age of a class of 39 students is 15
years. If the age of the teacher be included, then the average increases by3
months. Find the age of the teacher.
Sol. Total age of 39 persons = (39 x 15) years
=
585 years.
Average age of 40 persons= 15 yrs 3 months
= 61/4 years.
Total age of 40 persons = (_(61/4 )x 40) years= 610 years.
:. Age of the teacher = (610 - 585) years=25
years.
Ex. 12. The average weight of 10 oarsmen in a boat is increased by 1.8
kg when one of the crew, who weighs 53 kg is replaced by a new man. Find the
weight of the new
man.
Sol. Total weight
increased =(1.8 x 10) kg =18 kg.
:. Weight of the new man =(53 + 18) kg =71 kg.
Ex. 13. There were 35 students in a hostel. Due to the admission of 7
new students, ;he expenses of the mess were increased by Rs. 42 per day while
the average expenditure per head diminished by Rs 1. Wbat was the original
expenditure of the mess?
Sol. Let the
original average expenditure be Rs. x. Then,
42 (x - 1) - 35x=42 ó 7x= 84 ó x =12.
Original expenditure = Rs. (35 x 12) =Rs. 420. .
14. A batsman makes a score of 87 runs in the 17th inning and thus
increases his avg by 3. Find his average after 17th inning.
Sol. Let the average
after 17th inning = x.
Then,
average after 16th inning = (x - 3).
:. 16 (x - 3) + 87 = 17x or x =
(87 - 48) = 39.
Ex.15. Distance between two stations A and B is 778 km. A train covers
the journey from A to B at 84 km per hour and returns back to A with a uniform
speed of 56 km perhour. Find the average speed of the train during the whole
journey.
Sol. Required average speed = ((2xy)/(x+y)) km / hr
=(2 x 84 x 56)/(84+56)km/hr
=
(2*84*56)/140 km/hr
=67.2 km/hr.
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