diff --git a/11/solution.c b/11/solution.c
new file mode 100644
index 0000000..f5cc68b
--- /dev/null
+++ b/11/solution.c
@@ -0,0 +1,104 @@
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+
+typedef struct Monkey {
+    unsigned long business;
+    unsigned long items[32];
+    int count;
+    char op;
+    int val;
+    int test;
+    int true;
+    int false;
+} Monkey;
+
+int compar(const void *a, const void *b) {
+    const Monkey *m1 = a, *m2 = b;
+    if (m1->business < m2->business) return 1;
+    if (m1->business > m2->business) return -1;
+    return 0;
+}
+
+unsigned long monkey_business(Monkey *monkeys, int count, int steps, int part2) {
+    Monkey *copy = malloc(count * sizeof(Monkey));
+    memcpy(copy, monkeys, count * sizeof(Monkey));
+    monkeys = copy;
+
+    unsigned long lcm = 1;
+    if (part2)
+        for (int m = 0; m < count; m++)
+            lcm *= monkeys[m].test; // these are all primes
+
+    for (int s = 0; s < steps; s++) {
+        for (int m = 0; m < count; m++) {
+            for (int j = 0; j < monkeys[m].count; j++) {
+                unsigned long new = monkeys[m].items[j],
+                              val = monkeys[m].val;
+                if (!val) val = new;
+                monkeys[m].business++;
+
+                switch (monkeys[m].op) {
+                    case '+': new += val; break;
+                    case '*': new *= val; break;
+                }
+
+                if (part2) new %= lcm;
+                else new /= 3;
+
+                int target = monkeys[m].true;
+                if (new % monkeys[m].test)
+                    target = monkeys[m].false;
+
+                monkeys[target].items[monkeys[target].count++] = new;
+            }
+            monkeys[m].count = 0;
+        }
+    }
+
+    qsort(monkeys, count, sizeof(Monkey), compar);
+    unsigned long result = monkeys[0].business * monkeys[1].business;
+    free(monkeys);
+    return result;
+}
+
+int main(void) {
+    Monkey monkeys[9] = {0};
+    int i = -1;
+    char buf[64], *p;
+
+    while (fgets(buf, 64, stdin)) {
+        if (*buf == 'M') {
+            i++;
+        } else {
+            switch (buf[2]) {
+                case 'S':
+                    p = buf + 17;
+                    do {
+                        monkeys[i].items[monkeys[i].count] = atoi(++p);
+                        monkeys[i].count++;
+                    } while ((p = strchr(p, ',')));
+                    break;
+                case 'O':
+                    monkeys[i].op = buf[23];
+                    monkeys[i].val = atoi(buf + 25);
+                    break;
+                case 'T':
+                    monkeys[i].test = atoi(buf + 21);
+                    break;
+                case ' ':
+                    if (buf[7] == 't')
+                        monkeys[i].true = atoi(buf + 29);
+                    else
+                        monkeys[i].false = atoi(buf + 30);
+                    break;
+            }
+        }
+    }
+
+    i++;
+
+    printf("Silver: %lu\nGold: %lu\n",
+           monkey_business(monkeys, i, 20, 0),
+           monkey_business(monkeys, i, 10000, 0));
+}