 My approach :-

1. A nice number is one which has only ‘a’ and ‘b’ as its digits. And if sum of all digits of a nice number is also nice than it is known as very nice number. We have to make a number of n digits . Let ta be the number of ‘a’ digit and tb be the number of ‘b’ digit. Note for a nice number ta+tb = n .

2. So for ta equal to 0 to n we will find all the possible sum .

3. Then we will check if each sum is a nice number or not. if its nice than we have a total of nCta possible combination for a and b for a particular nice sum .

4. final ans will be total of nCta for all the nice sum.

My Code :-

``````#include <iostream>
#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
#define mod 1000000007

int checkNice(ll sum,ll a,ll b)
{
while(sum>0)
{	ll z = sum%10;
if(z!=a&&z!=b)
return 0;
sum/=10;
}
return 1;
}

ll fact(ll x )
{
ll ans=1;
while(x>0)
{
ans=(ans*x)%mod;
x--;
}
return ans ;
}

ll findNum(ll n,ll upto )
{
ll ans = 1;
for(ll g=n;g>upto;g--)
{
ans=(ans*g)%mod;
}
return ans ;
}

int main()
{
ll t; cin>>t;
for(int i=0;i<t;i++)
{
ll a,b,n;
cin>>a>>b>>n;

ll ans =0;
if(n==1)
{
cout<<2<<endl;
}
else
{
// n is the position that we have to fill with
// a and b

for(ll g=0;g<=n;g++)
{
ll ta,tb;
ta=g; tb=(n-g);
ll sum=0;
sum = ta*a+tb*b;
int flag = checkNice(sum,a,b);
//cout<<"flag is "<<flag<<endl;
if(flag==1)
{
ll fact1=n, fact2 = ta, fact3 =n-ta;

ll upto = (fact3>fact2)? fact3:fact2;

ll den = (fact3>fact2)? fact2:fact3;

//cout<<"upto and den are "<<upto<<" "<<den<<endl;

ll down = fact(den);

ll up = findNum(n,upto);

ans = (ans+up/down)%mod;
}
}
cout<<ans<<endl;
}
}

}
``````