diff --git a/Soyoung/5-1.c b/Soyoung/5-1.c
new file mode 100644
index 0000000..7e816aa
--- /dev/null
+++ b/Soyoung/5-1.c
@@ -0,0 +1,17 @@
+#include <stdio.h>
+
+int factorial(int n) {
+	if( n==0 || n== 1 ) {
+        return 1;
+    }
+    else {
+	    return n * factorial(n-1);
+    }
+}
+
+int main() {
+	int N;
+	scanf("%d", &N);
+	printf("%d", factorial(N));
+    return 0;
+}
\ No newline at end of file
diff --git a/Soyoung/5-2.c b/Soyoung/5-2.c
new file mode 100644
index 0000000..fd7fcf5
--- /dev/null
+++ b/Soyoung/5-2.c
@@ -0,0 +1,14 @@
+#include <stdio.h>
+
+char i_letter(char *S, int i) {  
+    return S[i-1];
+}
+
+int main() {
+    char S[1000];
+    scanf("%s", S); 
+    int i;
+    scanf("%d", &i);
+    printf("%c", i_letter(S, i));
+    return 0;
+}
\ No newline at end of file
diff --git a/Soyoung/5-3.c b/Soyoung/5-3.c
new file mode 100644
index 0000000..260462e
--- /dev/null
+++ b/Soyoung/5-3.c
@@ -0,0 +1,17 @@
+#include <stdio.h>
+
+int main() {
+    int T;
+    scanf("%d", &T); 
+    for(int i=0;i<T;i++) {
+        char S[1000];
+        scanf("%s", S);
+
+        int j=0;
+        while(S[j] != '\0') {
+            j++;
+        }
+        printf("%c %c\n", S[0], S[j-1]);
+    }
+    return 0;
+}
diff --git a/Soyoung/7_1.c b/Soyoung/7_1.c
new file mode 100644
index 0000000..ed8f7b0
--- /dev/null
+++ b/Soyoung/7_1.c
@@ -0,0 +1,23 @@
+#include <stdio.h>
+
+int main() {
+    int N;
+    scanf("%d", &N);
+    
+    int min = 1000000;  
+    int max = -1000000; 
+    
+    for (int i = 0; i < N; i++) {
+        int num;
+        scanf("%d", &num);
+        
+        if (num < min) {
+            min = num;
+        }
+        if (num > max) {
+            max = num;
+        }
+    }
+    printf("%d %d\n", min, max);
+    return 0;
+}
\ No newline at end of file
diff --git a/Soyoung/7_2.c b/Soyoung/7_2.c
new file mode 100644
index 0000000..ea8de6d
--- /dev/null
+++ b/Soyoung/7_2.c
@@ -0,0 +1,21 @@
+#include <stdio.h>
+
+int main() {
+    while (1) {   
+        int a, b;
+        scanf("%d %d", &a, &b);
+        
+        if (a == 0 && b == 0) {
+            break;
+        }
+        if (b % a == 0) {
+            printf("factor\n");
+        } else if (a % b == 0) {
+            printf("multiple\n");
+        } else {
+            printf("neither\n");
+        }
+    }
+    
+    return 0;
+}
diff --git a/Soyoung/7_3.c b/Soyoung/7_3.c
new file mode 100644
index 0000000..e8f7353
--- /dev/null
+++ b/Soyoung/7_3.c
@@ -0,0 +1,24 @@
+#include <stdio.h>
+
+int main() {
+	while(1) {
+        int a, b, c;
+        scanf("%d %d %d", &a, &b, &c);
+
+        a *= a; 
+        b *= b;
+        c *= c;
+
+        if(a == 0 && b == 0 && c == 0) {
+            break;
+        }
+        else if(((a + b) == c) || ((a + c) == b) || ((c + b) == a)) { //문제에서 c가 제일 큰 변이라는 보장이 없음
+            printf("right\n");
+        }
+        else {
+            printf("wrong\n");
+        }
+
+    }
+    return 0;
+}
diff --git a/Soyoung/C_study.c b/Soyoung/C_study.c
new file mode 100644
index 0000000..9d12d8a
--- /dev/null
+++ b/Soyoung/C_study.c
@@ -0,0 +1,20 @@
+#include <stdio.h>
+
+int main() {
+    int T;
+    scanf("%d", &T);
+    for(int t=0; t<T; t++) {  
+        int a, b, result;
+        scanf("%d %d", &a, &b);
+
+        a = a % 10;   
+        if (a == 0) {
+            result = 10;
+        }
+        else {
+            result = (a^((b % 4)+4)) % 10;
+        }
+        printf("%d\n", result);
+    }
+    return 0;
+} 
\ No newline at end of file
diff --git a/Soyoung/Summer_C_study b/Soyoung/Summer_C_study
new file mode 160000
index 0000000..3d186fb
--- /dev/null
+++ b/Soyoung/Summer_C_study
@@ -0,0 +1 @@
+Subproject commit 3d186fb78441d64674f870dc6afb3d79243b8ceb