In problem C, my approach was similar, but got WA in test-case 4. Can someone please check what is the error:

#include<bits/stdc++.h>

using namespace std;

int main(){
	long long int t,n,ans,m,k;
	cin>>t;
	while(t--){
		cin>>n>>m;
		int arr[10];
		arr[0]=m%10;
		for(int i=1;i<10;i++){
			arr[i]=(arr[i-1]+(arr[0]*(i+1))%10);
		}
		k=n/m;
		//cout<<k<<endl;
		ans=0;
		ans+=((k/10)*arr[9]);
		if(k>0)
		ans+=arr[(k%10)-1];
		cout<<ans<<endl;
	}
	return 0;
}