题意：
Motasem wants to cook n steaks (the steak consists of two faces) for his friends, and each face of the steak needs to be cooked for 5 minutes. Unfortunately, his pans can only fit 2 steaks at a time, and he only has k pans. If Motasem is cooking his steaks optimally, what is the minimum number of minutes he needs to cook all n steaks?
Input
A single line containing two space separated integers n,k (1≤n,k≤109) represent the number of steaks and pans respectively.
Output
A single integer represents the minimum time needed to cook all steaks
Example
inputCopy
3 1
outputCopy
15
题解：
一个平底锅可以放两张饼，一张饼需要烙两面，给n个饼和k个锅，烙熟需要5分钟，问最短多久可以烙完。
这道题我们可以把一张饼分离成两面，这样n个饼就变成了2n个面，因此每5分钟可以烙的数量为2k张面，接下来就很容易了。只是需要额外讨论一下当n/k<2时，10分钟即可搞定。
代码：
#include<stdio.h>
int main()
{
long long a,b;
scanf("%lld%lld",&a,&b);
if(a/b<2)printf("10\n");
else
{
if((a*2)%(b*2)!=0)printf("%lld\n",(a/b+1)*5);
else printf("%lld\n",(a/b)*5);
}
}