1. The average of 5 nos. is 46 and that of the first four of them is 45. The fifth number is
Sol:
The fifth number = total of 5 numbers - total of first four numbers
= 5 X 46 - 4 X 45
= 230 - 180 = 50
The 5th number is 50.
2. The average of n numbers is x.When 36 is substracted from two of the numbers, the new average becomes (x-8). The value of n is
Sol:
The average of 'n' numbers = x
Total of 'n' numbers = nx
According to this question,
nx – 36-36 = n(x-8)
nx – 72 = nx-8n
8n = 72
N = 72/8 = 9
The value of n is 9.
3. Four years ago the average age of A, B and C was 25 years. Five years ago the average age of and C was 20 years. A's present ages is
Sol:
Four years ago,
Average of A+B+C = 25 years
Now the age of A+B+C = 3 X 25 + 12 = 75 + 112 = 87 years.
Five years ago,
Average of B+C = 20 years
Now, the age of B+C = 2 X 20 + 10 = 40 + 10 = 50 years
A's present age = (A+B+C) is present ages – (B+C) is present ages
= 87 – 50 = 37 years.
4. A person goes to a place at the speed of 20 km/hr and returns at the speed of 12 km/hr. His average speed for the journey is
Sol:
Average speed = (2xy/x+y) km/hr.
X = 20km/hr. Y = 12 km/hr.
Average speed = 2 X 200 X 12 / 20 + 12
= 2 X 20 X 12 / 32
= 15 km/hr.
5. The average age of 20 boys in a class is 12 years. What will be the average age of the class of 5 new boys take admission whose average age is 7 years?
Sol:
Total ages of 20 boys = 20 X 12 = 240 years
Total ages of 5 new boys = 5 X 7 = 35 years
In the class, 25 boys total ages = 240 + 35 = 275 years
Average age of 25 boys = 275/25 = 11 years
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