Let d and s represent the number of daughters and sons respectively.

Then, we have :

d - 1 = s and 2 (s - 1) = d.

Solving these two equations, we get: d = 4, s = 3.

12+12*3 = 12 + 36 = 48

11 + 11*3 = 11 + 33 = 44

10 + 10*3 = 10 + 30 = 40

Let number of girls = x and number of boys = 3x.

Then, 3x + x = 4x = total number of students.

Thus, to find exact value of x, the total number of students must be divisible by 4.

Let x and y be the number of deer and peacocks in the zoo respectively. Then,

x + y = 80 ...(i) and

4x + 2y = 200 or 2x + y = 100 ...(ii)

Solving (i) and (ii), we get) x = 20, y = 60.

Number of cuts made to cut a roll into 10 pieces = 9.

Therefore Required number of rolls = (45 x 24)/9 = 120.

2*(AB) + 3*(AC) = 77

or, 6*(AB) + 9*(AC) = 231 (x 3)    ...................... (1)

And,

3*(AB) + 2*(AC) = 73

or, 6*(AB) + 4*(AC) = 146  (x 2) .......................... (2)

From, (1) - (2), we get,

5*(AC) = 231 - 146 = 85

or, (AC) = 85/5  = 17 tk

Now, replacing the value of (AC) in eq (1),

6*(AB) + 9*17 = 231

or, 6*(AB) = 231 - 153

or, (AB) = 78/6  =13 tk

Total runs scored = (36 x 5) = 180.

Let the runs scored by E be x.

Then, runs scored by D = x + 5; runs scored by A = x + 8;

runs scored by B = x + x + 5 = 2x + 5;

runs scored by C = (107 - B) = 107 - (2x + 5) = 102 - 2x.

So, total runs = (x + 8) + (2x + 5) + (102 - 2x) + (x + 5) + x = 3x + 120.

Therefore 3x + 120 =180 3X = 60 x = 20.

Since B and D are twins, so B = D.

Now, A = B + 3 and A = C - 3.

Thus, B + 3 = C - 3 D + 3 = C-3 C - D = 6.

Here,

The row is only accountable, as length is to be determined.

Now, the formula is= (n-1)*(Distance between two trees)

                                   = (10 - 1) * 2

                                   = 9*2

                                   = 18 m

And, additional 1 m is added on both side, thus (1 + 1) m is to be added with the resultant.

Thus, entire length of the garden is = 18 + (1 + 1)

x*1 + x*5 + x*10 = 480

x + 5x + 10x = 480

16x = 480

x = 480/16 = 30

Each denomination of notes is = 30

Total number of notes is = 3*30 = 90

3 years ago, their total age was = 81 - (3*3) = 72

Average of 3 person's age = 72/3

                                      = 24

Reverse method is useful here