diff --git a/Dynamic Programming/Investigation (Digit DP).cpp b/Dynamic Programming/Investigation (Digit DP).cpp
new file mode 100644
index 0000000..bb7c4a0
--- /dev/null
+++ b/Dynamic Programming/Investigation (Digit DP).cpp	
@@ -0,0 +1,78 @@
+// https://lightoj.com/problem/investigation
+
+#include <bits/stdc++.h>
+#define pb(x) push_back(x)
+#define all(x) (x).begin(), (x).end()
+#define rall(x) (x).rbegin(), (x).rend()
+#define F(w,x,y) for(ll w=x; w<y; w++)
+#define endl "\n"
+#define mod 1000000007
+typedef int ll;
+typedef long double ld;
+using namespace std;
+
+ll dp[11][90][90][2];
+ll m;
+
+ll solve(string s)
+{
+    memset(dp,0,sizeof(dp));
+
+    ll n=s.length();
+
+    dp[0][0][0][1]=dp[0][0][0][0]=1;
+
+    for(ll i=1; i<=n; i++)
+    {
+        for(ll j=0; j<m; j++)
+        {
+            for(ll k=0; k<m; k++)
+            {
+                ll ub=s[n-i]-'0';
+                for(ll digit=0; digit<=9; digit++)
+                {
+                    if(digit<=ub)
+                        dp[i][j][k][1]+=dp[i-1][(digit+j)%m][(digit+k*10)%m][1&(digit==ub)];
+                    dp[i][j][k][0]+=dp[i-1][(digit+j)%m][(digit+k*10)%m][0];
+                }
+            }
+        }
+    }
+
+    return dp[n][0][0][1];
+}
+
+void play()
+{
+    ll lvalue;
+    string r;
+    cin>>lvalue>>r>>m;
+    lvalue--;
+    string l=to_string(lvalue);
+
+    if(m==0 || m>=90)
+    {
+        cout<<0<<endl;
+        return;
+    }
+
+    cout<<solve(r)-solve(l)<<endl;
+}
+
+int main()
+{
+    //freopen("milkvisits.in","r",stdin);
+    //freopen("milkvisits.out","w",stdout);
+    ios_base::sync_with_stdio(false);
+    cin.tie(NULL);
+    //pre();
+    ll t;
+    t=1;
+    cin>>t;
+    F(i,1,t+1)
+    {
+        cout<<"Case "<<i<<": ";
+        play();
+    }
+    return 0;
+}