diff --git a/lib/vending-machine.js b/lib/vending-machine.js
index 4879b4b..7af9517 100644
--- a/lib/vending-machine.js
+++ b/lib/vending-machine.js
@@ -63,7 +63,7 @@ function getCoinsAvail () {
  */
 function sellProduct (totalPayable, coinsPaid, coinsAvail) {
   const cashPaid = coinsPaid.reduce( function(sum, c) { return sum + c }, 0);
-  const change = calculateChange(totalPayable, cashPaid);
+  const change = calculateChange(totalPayable, cashPaid, coinsAvail);
   // remove the coins we are about to return as change from the coinsAvail:
   setCoinsAvail(reduceCoinSupply(coinsAvail, change));
   return change;
diff --git a/tap-advanced-testing.md b/tap-advanced-testing.md
index e415c0e..30b9b5f 100644
--- a/tap-advanced-testing.md
+++ b/tap-advanced-testing.md
@@ -370,7 +370,8 @@ const reduceCoinSupply = vendingMachine.reduceCoinSupply;
 tap.test('reduceCoinSupply([200, 100, 50, 10, 1], [100, 50, 10]); returns [200, 1]', function (t) {
   const result = reduceCoinSupply([200, 100, 50, 10, 1], [100, 50, 10]);
   const expected = [200, 1];
-  t.deepEqual(result, expected);
+  t.deepEqual(result, expected,
+    'reduceCoinSupply reduces the coinsAvail by the coins given as change');
   t.end();
 });
 ```
@@ -447,7 +448,8 @@ tap.test('reduceCoinSupply remove more coins!', function (t) {
   ];
 
   const result = reduceCoinSupply(COINS, remove);
-  t.deepEqual(result, expected);
+  t.deepEqual(result, expected,
+    'reduceCoinSupply removes coins from supply of coinsAvail');
   t.end();
 });
 ```
@@ -499,9 +501,11 @@ tap.test('Check Initial Supply of Coins in Vending Machine', function (t) {
     2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
     1, 1, 1, 1, 1, 1, 1, 1, 1, 1
   ];
-  t.deepEqual(vendingMachine.getCoinsAvail(), COINS);
+  t.deepEqual(vendingMachine.getCoinsAvail(), COINS,
+    'vendingMachine.getCoinsAvail() gets COINS available in Vending Machine');
   vendingMachine.setCoinsAvail([1,2,3]);
-  t.deepEqual(vendingMachine.getCoinsAvail(), [1,2,3]);
+  t.deepEqual(vendingMachine.getCoinsAvail(), [1,2,3],
+    'vendingMachine.setCoinsAvail allows us to set the COINS available');
   t.end();
 });
 ```
@@ -519,7 +523,7 @@ you will see the last one fail:
 
 In `lib/vending-machine.js` file,
 after the `reduceCoinSupply` function definition,
-add the following variable declaration:
+add the following variable declaration and pair of functions:
 
 ```js
 // GOLBAL Coins Available Array. 10 of each type of coin.
@@ -534,7 +538,6 @@ let COINS = [
   1, 1, 1, 1, 1, 1, 1, 1, 1, 1
 ];
 
-
 /**
  * setCoinsAvail a "setter" for the COINS Array
  * @param {Array} coinsAvail the list of available coins
@@ -549,10 +552,11 @@ function setCoinsAvail (coinsAvail) {
 function getCoinsAvail () {
   return COINS;
 }
-
 ```
 
-We will _use_ the `COINS` array in the next step.
+We will _use_ the `COINS` array
+and both the "getter" and "setter"
+in the next step.
 For now, simply _export_ "getter" and "setter" so the test will pass:
 
 ```js
@@ -618,9 +622,12 @@ tap.test('sellProduct(215, [200, 100], COINS) returns [50, 20, 10, 5]', function
   const coinsPaid = [200, 100];
   const result = vendingMachine.sellProduct(215, coinsPaid, COINS);
   const expected = [50, 20, 10, 5];
-  t.deepEqual(result, expected);
+  t.deepEqual(result, expected,
+    'sellProduct returns the correct coins as change');
+
   // check that the supply of COINS Available in the vendingMachine was reduced:
-  t.deepEqual(vendingMachine.getCoinsAvail(), reduceCoinSupply(COINS, result));
+  t.deepEqual(vendingMachine.getCoinsAvail(), reduceCoinSupply(COINS, result),
+    'running the sellProduct function reduces the COINS the vending machine');
   t.end();
 });
 ```
@@ -653,7 +660,7 @@ If you run the tests:
 node test/vending-machine.test.js
 ```
 
-you will see the last one fail:
+you will see the last one _fail_:
 
 ![sellProduct-test-fail](https://user-images.githubusercontent.com/194400/47678596-1bb44880-dbba-11e8-8927-892149a201bd.png)