1005_地球人口承载力

#include<iostream>
#include<cstdio>
using namespace std;
int main()
{
  int x,a,y,b;
  double z;
  cin>>x>>a>>y>>b;

  /*
  设地球现有资源now,每年新增up,每年消耗down
  若可持续发展
  year*up = z*year*down
  z = up/down

  now + a*up = a*x*down
  now + b*up = b*y*down

  两式相减

  a*up-b*up = a*x*down-b*y*down
  up(a-b) = down(ax - by)
  up/down = (ax - by)/(a-b)

  */
  z=(1.0*a*x-b*y)/(1.0*a-b);
  printf("%.2f",z);

  return 0;
}