diff --git a/.gitignore b/.gitignore
index 3d45206..6dc692f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
 **/node_modules
 **/flow-typed
 **/flow-coverage
+**/out
diff --git a/chapter02/package.json b/chapter02/package.json
index 4114359..e0ce03d 100644
--- a/chapter02/package.json
+++ b/chapter02/package.json
@@ -37,6 +37,7 @@
             "prefer-const": "error"
         }
     },
+    "eslintIgnore": ["**/out/*.js"],
     "flow-coverage-report": {
         "concurrentFiles": 1,
         "excludeGlob": ["node_modules/**"],
diff --git a/chapter03/package.json b/chapter03/package.json
index 7124375..fee6f42 100644
--- a/chapter03/package.json
+++ b/chapter03/package.json
@@ -15,10 +15,7 @@
     "author": "Federico Kereki",
     "license": "ISC",
     "babel": {
-        "presets": [
-            "env",
-            "flow"
-        ]
+        "presets": ["env", "flow"]
     },
     "eslintConfig": {
         "parserOptions": {
@@ -31,34 +28,21 @@
             "node": true,
             "es6": true
         },
-        "extends": [
-            "eslint:recommended",
-            "plugin:flowtype/recommended"
-        ],
-        "plugins": [
-            "babel",
-            "flowtype"
-        ],
+        "extends": ["eslint:recommended", "plugin:flowtype/recommended"],
+        "plugins": ["babel", "flowtype"],
         "rules": {
             "no-console": "off",
             "no-var": "error",
             "prefer-const": "error"
         }
     },
+    "eslintIgnore": ["**/out/*.js"],
     "flow-coverage-report": {
         "concurrentFiles": 1,
-        "excludeGlob": [
-            "node_modules/**"
-        ],
-        "includeGlob": [
-            "src/**/*.js"
-        ],
+        "excludeGlob": ["node_modules/**"],
+        "includeGlob": ["src/**/*.js"],
         "threshold": 90,
-        "type": [
-            "text",
-            "html",
-            "json"
-        ]
+        "type": ["text", "html", "json"]
     },
     "prettier": {
         "tabWidth": 4,
diff --git a/chapter03/src/doroundmath.js b/chapter03/src/doroundmath.js
new file mode 100644
index 0000000..dabe98a
--- /dev/null
+++ b/chapter03/src/doroundmath.js
@@ -0,0 +1,11 @@
+/* @flow */
+"use strict";
+
+const RM = require("./roundmath.js");
+const { multR, divR } = require("./roundmath.js");
+
+console.log(RM.addR(12.348, 4.221)); // 16.57
+console.log(multR(22.9, 12.4)); // 283.96
+console.log(divR(22, 7)); // 3.14
+
+console.log(RM.changeSign(0.07)); // error; RM.changeSign is not a function
diff --git a/chapter03/src/roundmath.js b/chapter03/src/roundmath.js
new file mode 100644
index 0000000..47a2079
--- /dev/null
+++ b/chapter03/src/roundmath.js
@@ -0,0 +1,30 @@
+/* @flow */
+"use strict";
+
+const roundToCents = (x: number): number => Math.round(x * 100) / 100;
+
+const changeSign = (x: number): number => -x;
+
+const addR = (x: number, y: number): number => roundToCents(x + y);
+
+const subR = (x: number, y: number) => addR(x, changeSign(y));
+
+const multR = (x: number, y: number): number => roundToCents(x * y);
+
+const divR = (x: number, y: number): number => {
+    if (y === 0) {
+        throw new Error("Divisor must be nonzero");
+    } else {
+        return roundToCents(x / y);
+    }
+};
+
+/*
+    NOTES:
+    1. Exports are all together, at the end, per convention
+    2. roundToCents and changeSign are not exported, on purpose
+*/
+exports.addR = addR;
+exports.subR = subR;
+exports.multR = multR;
+exports.divR = divR;