diff --git a/001_syntax/006_object.js b/001_syntax/006_object.js
index 1ac6fff..26c1ee1 100644
--- a/001_syntax/006_object.js
+++ b/001_syntax/006_object.js
@@ -82,7 +82,6 @@ function isDuplicate(array) {
   for (let i = 0; i < array.length; i++) {
     //配列の要素をvalueに代入
     const value = array[i];
-
     //objに配列要素があったら
     if (obj[value]) {
       return true;
diff --git a/002_class/003_data.js b/002_class/003_data.js
index 96869ff..f289c65 100644
--- a/002_class/003_data.js
+++ b/002_class/003_data.js
@@ -121,10 +121,12 @@ class List {
    */
   filter(target) {
     // TODO:
-    let result = new List(this.data);
-    for (let i = 0; i <= result.length; i++) {
-      if (result[i] === target) {
-        result.splice(i, 1);
+    let result = new List(this.data); //Listインスタンスを作成
+    //自身で作成したsizeを使用
+    for (let i = 0; i <= result.size; i++) {
+      //自身で作成したindexを使用
+      if (result.index(i) === target) {
+        result.remove(i); //自身で作成したremoveを使用
       }
     }
     return result;
diff --git a/003_practice/001_easy.js b/003_practice/001_easy.js
index 27567dd..99fed97 100644
--- a/003_practice/001_easy.js
+++ b/003_practice/001_easy.js
@@ -12,7 +12,7 @@
 
 function length(str) {
   let ans = 0;
-  for (let i = 1; i <= str.length; i++) {
+  while (str[ans]) {
     ans++;
   }
   return ans;
@@ -30,9 +30,12 @@ function length(str) {
  *
  */
 function reverse(str) {
-  let strArray = [...str];
-  strArray.reverse();
-  return strArray.join("");
+  let newStr = "";
+  //後ろ側から取り出して並べる
+  for (let i = str.length - 1; i >= 0; i--) {
+    newStr += str[i];
+  }
+  return newStr;
 }
 
 /**
@@ -48,7 +51,12 @@ function reverse(str) {
  */
 
 function findIndex(str, char) {
-  return str.indexOf(char);
+  for (let i = 0; i < str.length - 1; i++) {
+    if (str[i] === char) {
+      return i;
+    }
+  }
+  return -1;
 }
 
 /**
@@ -103,13 +111,13 @@ function sum(array) {
 function average(array) {
   if (array.length === 0) {
     return 0;
-  } else {
-    const sum = array.reduce((pre, current) => {
-      return pre + current;
-    }, 0);
-    const average = sum / array.length;
-    return Math.floor(average);
   }
+
+  let average = 0;
+  for (let i = 0; i < array.length; i++) {
+    average += array[i];
+  }
+  return Math.floor(average / array.length);
 }
 
 /**
@@ -125,7 +133,7 @@ function average(array) {
  */
 
 function concat(a, b) {
-  return a.concat(b);
+  return [...a, ...b];
 }
 
 /**
@@ -141,7 +149,11 @@ function concat(a, b) {
  */
 
 function size(array) {
-  return array.length;
+  let ans = 0;
+  for (let value of array) {
+    ans++;
+  }
+  return ans;
 }
 
 /**
@@ -200,13 +212,13 @@ function seq(num) {
  */
 
 function omitSeq(num) {
-  let ans = 0;
+  let ans = 1;
   let result = [];
   while (ans <= num) {
     result.push(ans);
-    ans += 1;
+    ans += 2;
   }
-  return result.filter((value) => value % 2 == 1);
+  return result;
 }
 
 /**
diff --git a/003_practice/002_medium.js b/003_practice/002_medium.js
index 026b239..c75df76 100644
--- a/003_practice/002_medium.js
+++ b/003_practice/002_medium.js
@@ -33,19 +33,20 @@ function rotate(str, num) {
  *
  */
 function removeVowels(str) {
-  let strArray = [...str];
-  strArray.forEach((value) => {
+  let newStr = "";
+
+  for (let i = 0; i < str.length; i++) {
     if (
-      value === "a" ||
-      value === "e" ||
-      value === "i" ||
-      value === "o" ||
-      value === "u"
+      str[i] !== "a" &&
+      str[i] !== "e" &&
+      str[i] !== "i" &&
+      str[i] !== "o" &&
+      str[i] !== "u"
     ) {
-      strArray.splice(strArray.indexOf(value), 1, "");
+      newStr += str[i];
     }
-  });
-  return strArray.join("");
+  }
+  return newStr;
 }
 
 /**
@@ -79,14 +80,12 @@ function countStr(s1, s2) {
  */
 
 function isPalindrome(str) {
-  const strArray = [...str];
-  const strReverse = [...str].reverse();
-
-  if (strArray.toString() === strReverse.toString()) {
-    return true;
-  } else {
-    return false;
+  for (let i = 0; i < str.length / 2; i++) {
+    if (str[i] !== str[str.length - 1 - i]) {
+      return false;
+    }
   }
+  return true;
 }
 
 /**
@@ -104,13 +103,16 @@ function isPalindrome(str) {
  *
  */
 function isPrime(num) {
-  if (num === 2 || num === 3) {
-    return true;
-  } else if (num === 1 || num % 2 === 0 || num % 3 === 0) {
+  //numが2の場合
+  if (num === 1) {
     return false;
-  } else {
-    return true;
   }
+  for (let i = 2; i < num; i++) {
+    if (num % i === 0) {
+      return false;
+    }
+  }
+  return true;
 }
 
 /**
@@ -129,32 +131,15 @@ function isPrime(num) {
  *
  */
 function sumWithout4andNext(array) {
-  //空の配列をsumArrayを定義
-  let sumArray = [];
+  let total = 0;
 
-  //array分ループを回す
   for (let i = 0; i < array.length; i++) {
-    //array[1]が4だったら
-    if (array[i] === 4) {
-      //sumArrayに0をpush
-      sumArray.push(0);
-
-      //次の要素も4だった場合
-      if (array[i + 1] === 4) {
-        //次の要素も0にする
-        array[i] = 0;
-      } else {
-        array[i + 1] = 0;
-      }
-    } else {
-      sumArray.push(array[i]);
+    if (array[i - 1] !== 4 && array[i] !== 4) {
+      total += array[i];
     }
   }
 
-  const sum = sumArray.reduce((pre, current) => {
-    return pre + current;
-  });
-  return sum;
+  return total;
 }
 
 module.exports = {