For infinite solution
a₁/a₂ = b₁/₂ = c₁/c₂
? K/12 = 3/K = (-K + 3)/ -K
? K/12 = 3/K
? K² = 36
? K = ?36 = 6

Let the number of correct answer be x and number of wrong answer be y.
Then, 4x - y = 200 ..(i)
and x + y = 200 ..(ii)

On adding Eqs. (i) and (ii), we get
4x - y = 200
x + y = 200
----------------
5x = 400
? x = 80

Let us assume N be the given number.
According to the question,
Sum of third , fourth and fifth part of a number exceeds half of the number by 34.
N/3 + N/4 + N/5 - 34 = N/2
N/3 + N/4 + N/5 - N/2 = 34
(20N + 15N + 12N - 30N)/60 = 34
17N/60 = 34
N = 34 x 60/17
N = 2 x 60
N = 120

Let us assume the number of Hens are H and Number of Goats are G.
According to question,
The total number of animal heads are 81. Since one animal has one head.
Number of head of animals = Number of total animals
H + G = 81..........................(1)
The total number of animal legs are 234. Since hen's have 2 legs and goat's have 4 legs.
2H + 4G = 234...................(2)
Multiply 2 with equation (1) and subtract from equation (2), we will get.
? 2H + 4G - 2 x (H + G ) = 234 - 2 x 81
? 2H + 4G - 2H - 2G = 234 - 162
? 2G = 72
? G = 36
Put the value of G in equation (1), we will get.
H + 36 = 81
H = 81 - 36
H = 45
Krishna have number of goats = G = 36

Let us assume the present age of father = F year and Son's present age = S years
According to question, 5 years ago,
Father's age = F - 5 and Son's age = S - 5.
According to the question,
The age of the father 5 years ago was 5 times the age of his son.
F - 5 = 5(S - 5)
F - 5 = 5S - 25.....................(1)
At present the father's age is 3 times that of his son.
F = 3S.................................(2)
Put the value of F from equation (2) in equation (1), we will get
? 3S - 5 = 5S - 25
? 25 - 5 = 5S - 3S
? 20 = 2S
? 10 = S
? S = 10.
Put the value of S in Equation (2). we will get,
F = 3S = 3 x 10 = 30
So the present Age of Father = 30.

Let us assume the digits of the original number are unit's digit a and ten's digit b.
The Original Number will be 10a + b.
After interchanging the digits the new number will be 10b + a.
According to question,
The number obtained by interchanging the two digits of a two-digit number is lesser than the original number by 54.
New Number = Original Number - 54
10b + a = 10a + b - 54
? 10b + a - 10a - b = -54
? 9b - 9a = -54
? a - b = 6....................................(1)
Again according to question,
Sum of the digits of original number = 10
a + b = 10..................................................(2)
Add the equation (1) and (2), we will get
a - b + a + b = 10 + 6
2a = 16
a = 8
Put the value of a in Equation (2) , we will get
8 + b = 10
b = 10 - 8
b = 2
Put the value of a and b for original number, we will get
10a + b = 10 x 8 + 2 = 80 + 2 = 82

Let us assume the certain Number is A.
According to question,
63A = 3834 + 36A
63A - 36A = 3834
? 27A = 3834
? A = 142

Suppose that the Ravi present age is A years.
According to the given question,
A/4 + A/5 + A/3 + 13 = A
? (15A + 12A + 20A)/60 = A - 13
? 47A = 60A - 780
?60A - 47A = 780
?13A = 780
? A = 780/13 = 60 years

Method 1
Let us assume the present age of mother be M and daughter be D.
According to question,
Ratio of present age of mother and daughter = 7 : 1
M / D = 7 : 1
? M / D = 7 / 1
? M / D = 7
? M = 7D ..........................(1)
Four years ago the age of mother = M - 4
Four years ago the age of daughter = D - 4
Again according to question,
Four years ago ratio of mother age and daughter age = 19 : 1
? (M - 4 ) / (D - 4) = 19 : 1
? (M - 4 ) / (D - 4) = 19
? (M - 4 ) = 19(D - 4)
? M - 4 = 19D - 76 .....................(2)
Put the value of M from equation (1) in above equation (2), we will get
7D - 4 = 19D - 76
? 76 - 4 = 19D - 7D
? 72 = 12D
? 12D = 72
? D = 72/12
? D = 6
Put the vale of D in equation (1), we will get the present age of mother.
Present age of Mother M = 7D = 7 x 6
Present age of Mother M = 42
? mother's age after 4 yrs = 42 + 4 = 46 yrs

Method 2
Let us assume the ratio factor is x.
According to question,
Ratio of present age of mother and daughter = 7 : 1
Then present age of mother = 7x and present age of daughter = x
Four years ago the age of mother = 7x - 4
Four years ago the age of daughter = x - 4
Again according to question,
Four years ago ratio of mother age and daughter age = 19 : 1
? (7x - 4 ) / (x - 4) = 19 : 1
? (7x - 4 ) / (x - 4) = 19
? (7x - 4 ) = 19(x - 4)
? 7x - 4 = 19x - 76
? 76 - 4 = 19x - 7x
? 72 = 12x
? 12x = 72
? x = 6
Present age of Mother = 7x = 7 x 6 = 42
? mother's age after 4 yrs = 42 + 4 = 46 yrs

Let the numbers be x and y.
According to question,
The ratio of two numbers is 4:7.
? x/y = 4/7
? 7x = 4y
? 7x - 4y = 0 ........................(1)
Again According to question,
If each of those numbers increased by 30, their ratio will become 5:8
(x + 30) / (y + 30) = 5/8
? 8(x + 30) = 5(y + 30)
? 8x + 240 = 5y + 150
? 8x - 5y = -90 ..........................(2)
Multiply 5 with equation (1) , we will get.
35x - 20y = 0 ..................(3)
Multiply 4 with equation (2), we will get.
32x - 20y = -360 ............(4)
Subtracts the Equation (4) from Equation (3). we will get,
35x - 20y - (32x - 20y) = 0 - (-360)
? 35x - 20y - 32x + 20y = 360
? 3x = 360
? x = 120
Put the value of x in equation (1) to get the value of y.
7x - 4y = 0
? 7(120) - 4y = 0
? 840 - 4y = 0
? 4y = 840
? y = 210
? Average of the numbers = (x + y)/2
put the vale of x and y.
? Average of the numbers = (120 + 210)/2
? Average of the numbers = 330/2
? Average of the numbers = 165