1, let the number of boys x and the number of girls y, then:

X+Y=2300

1.25 x+0.75y = 2300 * 1. 15 .

The solution is X= 1840. The existing number of boys is 1.25X=2300.

2. If the values of x and y are opposite, then y =-x .. Substitute into the equation:

X+2X=-2a

2X-5X=a+6

The solution is a=6.

3. If the solutions of the two equations are the same, then four straight lines intersect at the same point. (number-shape combination method)

Then 2x+5y =-6; The intersection (2, -2) of 3X-5Y= 16 can be obtained by solving two sets of equations.

So substitute the intersection into the other two equations and you get:

2a+2b=-4

2b-2a=-8

The solution is a= 1 and b=-3.

So the 20 12 power of (2a+b) is equal to the 20 12 power of (-1) is equal to 1.

For masquerade, because boys can't see themselves, X- 1 should be set as the equation when all boys are X.

Similarly, girls can't see themselves, and y must be reduced by 1.

Let all boys be X and all girls be Y.

Then Y=2(X- 1)+ 1.

3/5(Y- 1)=X

X=6，Y= 1 1。