#include<bits/stdc++.h>
using namespace std;
#define Min(a, b) a < b ? a : b
int main(){
    int n, a, p, m = INT_MAX;
    cin >> n;
    for(int i = 0; i < 3; i++){
        cin >> a >> p;
        m = Min(m, ceil(1.0 * n / a) * p);
    }
    cout << m;
    return 0;
}

没必要的繁琐写法

#include <bits/stdc++.h>
using namespace std;
struct p {
    int price;
    int amount;
    int need;
    int cost;
}pencil[3];
bool cmp(p a, p b) {
    return a.cost < b.cost;
}
int main() {
    int n;
    cin >> n;
    for(int i = 0; i < 3; i++) {
        cin >> pencil[i].amount >> pencil[i].price;
        pencil[i].need = ceil((double)n / pencil[i].amount);
        pencil[i].cost = pencil[i].need * pencil[i].price;
    }
    sort(pencil, pencil + 3, cmp);
    cout << pencil[0].cost;
    return 0;
}

更新：优化码风

依题意模拟即可

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


int main()
{
	int n,a,b,i,j,k,low=100000001;
	cin>>n;
	for(i=1;i<=3;i++){
		cin>>a>>b;
		for(j=1;;j++){
			if(a*j>=n){
				k=b*j;
				break;
			}
			else continue;
		}
		if(k<=low) low=k;
	}
	cout<<low<<endl;
}

#include<cstdio>
#include<cmath>
using namespace std; 
int main(){	
float n_student=0,n_pencil=0,money,smoney=0,money_temp=0;int i=1;
	scanf("%f",&n_student);
	scanf("%f%f",&n_pencil,&money);
	smoney=ceil(n_student/n_pencil)*money;
	while(i<3){
		scanf("%f%f",&n_pencil,&money);
		money_temp=ceil(n_student/n_pencil)*money;
		if(money_temp<smoney) 
smoney=money_temp;
		i++;
	}
	printf("%.0f",smoney);
	return 0;
}

使用ceil进行一个不为整数时产生进位

from math import ceil
n=int(input())
a1,p1=map(int,input().split())
money=ceil(n/a1)*p1
a2,p2=map(int,input().split())
money=min(money,ceil(n/a2)*p2)
a3,p3=map(int,input().split())
print(min(money,ceil(n/a3)*p3))