diff --git a/semester3/exercises/exercise2.13.c b/semester3/exercises/exercise2.13.c
new file mode 100644
index 0000000..61a3a39
--- /dev/null
+++ b/semester3/exercises/exercise2.13.c
@@ -0,0 +1,37 @@
+#include <math.h>
+#include <stdio.h>
+#include <time.h>
+
+void printTime(clock_t end, clock_t start) {
+  printf("Took %lf to calculate.\n", ((double)(end - start)) / CLOCKS_PER_SEC);
+}
+
+void calculate(long long N) {
+  clock_t end, start;
+
+  start = clock();
+
+  for (int i = 1, root = (int)floor(sqrt((double)N)); i < root; i++) {
+    if (N % i == 0) {
+      end = clock();
+      printTime(end, start);
+      printf("%lld is not prime\n", N);
+      return;
+    }
+  }
+
+  end = clock();
+  printTime(end, start);
+  printf("%lld is prime\n", N);
+  return;
+}
+
+int main(void) {
+  long long test[] = {1048575, 1099511627775};
+  for (int i = 0; i < 2; i++) {
+    calculate(test[i]);
+    printf("\n");
+  }
+
+  return 0;
+}
diff --git a/semester3/exercises/exercise2.6.c b/semester3/exercises/exercise2.6.c
new file mode 100644
index 0000000..b62bcbc
--- /dev/null
+++ b/semester3/exercises/exercise2.6.c
@@ -0,0 +1,116 @@
+#include <stdio.h>
+#include <time.h>
+
+int counter = 0;
+
+void getTime(int N, clock_t start_t, time_t end_t) {
+  double runningTime;
+  runningTime = ((double) (end_t - start_t)) / CLOCKS_PER_SEC;
+
+  printf("%d: Took %lf sec\n", counter++, runningTime);
+}
+
+void test(int N) {
+  clock_t start_t;
+  clock_t end_t;
+  int sum;
+
+  // 1
+  sum = 0;
+
+  start_t = clock();
+
+  for (int i = 0; i < N; i++) {
+    sum++;
+  }
+
+  end_t = clock();
+
+  getTime(N, start_t, end_t);
+
+  // 2
+  start_t = clock();
+  sum = 0;
+
+  for (int i = 0; i < N; i++) {
+    for (int j = 0; j < N; j++) {
+      sum++;
+    }
+  }
+
+  end_t = clock();
+
+  getTime(N, start_t, end_t);
+
+  // 3
+  start_t = clock();
+  sum = 0;
+
+  for (int i = 0; i < N; i++) {
+    for (int j = 0; j < N * N; j++) {
+      sum++;
+    }
+  }
+
+  end_t = clock();
+  getTime(N, start_t, end_t);
+
+  // 4
+  start_t = clock();
+
+  sum = 0;
+
+  for (int i = 0; i < N; i++) {
+    for (int j = 0; j < i; j++) {
+      sum++;
+    }
+  }
+
+  end_t = clock();
+  getTime(N, start_t, end_t);
+
+  // 5
+  start_t = clock();
+
+  sum = 0;
+
+  for (int i = 0; i < N; i++) {
+    for (int j = 0; j < i * i; j++) {
+      for (int k = 0; k < j; k++) {
+        sum++;
+      }
+    }
+  }
+
+  end_t = clock();
+  getTime(N, start_t, end_t);
+
+  // 6
+  start_t = clock();
+
+  sum = 0;
+
+  for (int i = 1; i < N; i++) {
+    for (int j = 1; j < i * i; j++) {
+      if (j % i == 0) {
+        for (int k = 0; k < j; k++) {
+          sum++;
+        }
+      }
+    }
+  }
+
+  end_t = clock();
+  getTime(N, start_t, end_t);
+}
+
+int main(void) {
+  int times[] = {5, 10, 100, 250};
+
+  for (int i = 0; i < 4; i++) {
+    counter = 0;
+    printf("N = %d\n\n", times[i]);
+    test(times[i]);
+    printf("\n");
+  }
+}