二分
LA上的題目居然和原題有差別！變成了多組數據
下面這個程序是按原題寫的

#include <iostream>
#include <cstdio>
#include <algorithm>

int main() {
  int n, t;
  std::cin >> n >> t;
  
  double d[1000], s[1000];
  for (int i = 0; i < n; i++)
    std::cin >> d[i] >> s[i];
  
  double l = -*std::min_element(s, s + n), r = 1001000
 
;
  while (l + 2e-10 <= r) {
    double mid = l + (r - l) / 2, t_ = 0;
    
    for (int i = 0; i < n; i++)
      t_ += d[i] / (s[i] + mid);
    
    if (t_ < t)
      r = mid;
    else
      l = mid;
  }
  printf("%.9lf\n", l);
  return 0;
}

LA 8043. ACM-ICPC World Finals 2017 E. Need for Speed