I. 【习1.5】地球人口承载力估计

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75.00

#include<bits/stdc++.h>
using namespace std;

int main(){
	//原有S, 每年产生s，每一亿人每年消耗 o 
	//①S + a * s = x * a * o
	//②S + b * s = y * b * o
	//①- ②：a * s - b * s = x * a * o - y * b * o;
	//转化：(a - b) * s = (x * a - y * b) * o;
	//(a - b) * s / o= (x * a - y * b) * o / o
	//=(a - b) * s / o= (x * a - y * b)
	//(a - b) / (a - b) * s / o= (x * a - y * b) / (a - b)
	//s / o = (x * a - y * b) / (a - b);
	double x, y, a, b;
    cin >> x >> a >> y >> b;
    printf("%.2lf", (x * a - y * b) / (a - b));
	return 0;
}