看题是以为是动态规划，结果是贪心

#include <bits/stdc++.h>
using namespace std;
int n, t;
double ans;
struct Node
{
	int w, v;
}a[105];
bool cmp(Node x,Node y)
{
	return x.v * y.w > x.w * y.v;
}
int main()
{
    cin >> n >> t;
    for (int i = 1; i <= n; i++)
        cin >> a[i].w >> a[i].v;
    sort (a + 1, a + n + 1, cmp);
    for (int i = 1; i <= n; i++)
    {
        if (a[i].w <= t)
        {
            ans += a[i].v;
            t -= a[i].w;
        }
		else
        {
			ans += a[i].v * t * 1.0 / (a[i].w * 1.0);
			break;
		}
    }
    cout << fixed << setprecision(2) << ans << endl;
    return 0;
}

#include<bits/stdc++.h>
int n,T;
double ans;
struct node
{
    int m,v;
}a[101];
bool cmp(node &x,node &y)
{
    return x.v*y.m>=x.m*y.v;
}
int main()
{
    std::cin>>n>>T;
    for(int i=1;i<=n;i+=1)std::cin>>a[i].m>>a[i].v;
    std::sort(a+1,a+n+1,cmp);
    for(int i=1;i<=n;i+=1)
    {
        int x=std::min(T,a[i].m);
        ans+=(double)x/a[i].m*a[i].v;
        T-=x;
    }
    printf("%.2f\n",ans);
}

#include <iostream>
#include <cstdio>
#include <algorithm>
struct coin{int m,v;}coins[105];
int yasuo(coin a,coin b){
    return a.v*b.m>b.v*a.m;} 
int main(){
    int n,t;double x,ans=0; std::cin>>n>>t;
    for(int i=0;i<n;i++)std::cin>>coins[i].m>>coins[i].v;
    std::sort(coins,coins+n,yasuo);
    for(int i=0;i<n;i++){
        x=std::min(t,coins[i].m);
        ans+=x*coins[i].v/coins[i].m;
        t-=x;}
    printf("%.2f",ans); return 0;}