The ratio of the numbers of boys and girls in a school was 5 : 3. Some new boys and girls were admitted to the school, in the ratio 5 : 7. At this, the total number of students in the school became 1200 and the ratio of boys to girls changed to 7 : 5. The number of students in the school before new admissions was = ?

1700
2720
3900
4960

Answer:- 1

Explanation:-

Solution:

\begin{aligned} A : B \\ 5 : 3 \\ Let 5 x : 3 x = 8 x \\ \Rightarrow New comers \\ 5 y : 7 y = 12 y \\ ∴ 8 x + 12 = 1200 \\ \Rightarrow 2 x + 3 y = 300...... (i) \\ Again, \frac{5 x + 5 y}{3 x + 7 y} = \frac{7}{5} \\ 25 x + 25 y = 21 x + 49 y \\ \Rightarrow 4 x - 24 y = 0 \\ \Rightarrow 4 x = 24 y \\ \Rightarrow x = 6 y . . . . . . . . . . . (i i) \\ ∴ From equation . . . . . . . (i) \\ 12 y + 3 y = 300 \\ y = 20 \\ ∴ x = 6 \times 20 = 120 \end{aligned}

$\eqalign{ & \,\,\,\,\,\,\,\,\,\,\,{\text{A}}:{\text{B}} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,5:3 \cr & {\text{Let }}5x:3x = 8x \cr & \Rightarrow {\text{New comers}} \cr & 5y:7y = 12y \cr & \therefore 8x + 12 = 1200 \cr & \Rightarrow 2x + 3y = 300......(i) \cr & {\text{Again,}}\frac{{5x + 5y}}{{3x + 7y}} = \frac{7}{5} \cr & 25x + 25y = 21x + 49y \cr & \Rightarrow 4x - 24y = 0 \cr & \Rightarrow 4x = 24y \cr & \Rightarrow x = 6y...........(ii) \cr & \therefore {\text{From equation }}.......(i) \cr & 12y + 3y = 300 \cr & y = 20 \cr & \therefore x = 6 \times 20 = 120 \cr}$ The number of students initially
8x = 8 × 120 = 960