diff --git a/AcWing/878/878.cpp b/AcWing/878/878.cpp
new file mode 100644
index 00000000..320c728f
--- /dev/null
+++ b/AcWing/878/878.cpp
@@ -0,0 +1,40 @@
+#include <iostream>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+int n, a, b, m, x, y;
+
+int exgcd(int a, int b, int &x, int &y) {
+    if (!b) {
+        x = 1, y = 0;
+        return a;
+    }
+
+    int d = exgcd(b, a % b, y, x);
+
+    y -= a / b * x;
+
+    return d;
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+
+    cin >> n;
+
+    while (n--) {
+        cin >> a >> b >> m;
+
+        int d = exgcd(a, m, x, y);
+
+        if (b % d) {
+            cout << "impossible" << endl;
+        } else {
+            cout << 1ll * b / d * x % m << endl;
+        }
+    }
+
+    return 0;
+}