Question 1197222
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@MathLover1's solution is incorrect.


Notice she's dividing by 31, but only added up 6 items.
Instead she should be adding up 31 items to represent the 31 days of October


Each day will have a certain balance associated with it. It might help to think of a calendar view as shown in this video here
<a href = "https://www.youtube.com/watch?v=3TSxhuuLDc0">https://www.youtube.com/watch?v=3TSxhuuLDc0</a>


I do NOT recommend using this method as it's tedious busywork. 
There are many repeated values that it would be a pain to write/type out over and over. 


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A shortcut is to do a weighted average.


Here's what the chart would look like to show the daily balance for any given period in October.<table border = "1" cellpadding = "5"><tr><td>Timespan</td><td>Number of days</td><td>Balance</td><td>Notes</td></tr><tr><td>Oct 1st to Oct 2nd </td><td>2</td><td>120</td><td>Starting balance</td></tr><tr><td>Oct 3rd to Oct 6th</td><td>4</td><td>120+50 = 170</td><td>Cash Advance of $50 (on Oct 3rd)</td></tr><tr><td>Oct 7th to Oct 9th</td><td>3</td><td>170-75 = 95</td><td>Payment of $75 (Oct 7th)</td></tr><tr><td>Oct 10th to Oct 15th</td><td>6</td><td>95+29.59 = 124.59</td><td>Purchase of $29.59 (Oct 10th)</td></tr><tr><td>Oct 16th to Oct 24th</td><td>9</td><td>124.59-30 = 94.59</td><td>Credit of $30.00 (happened Oct 16th)</td></tr><tr><td>Oct 25th to Oct 31st</td><td>7</td><td>94.59 + 121.60 = 216.19</td><td>Purchase of $121.60 (happened Oct 25th)</td></tr></table>


For the first 2 days of October, the balance is $120 (the previous month's balance)
Then on Oct 3rd, there's a cash advance of $50
This amount is loaned to the customer (it's basically a line of credit)
Therefore, the balance to be repaid increases to 120+50 = 170 dollars. This balance stays in effect from Oct 3rd to Oct 6th as the chart mentions.


On Oct 7th, a payment of $75 is made. This reduces the balance to 170-75 = 95 dollars. 
This balance of $95 is in effect from Oct 7th to Oct 9th which is a period of 3 days.


This process is carried out until we arrive at Oct 31st. 
Payments and credits reduce the balance; while purchases and cash advances increase the balance.


Here's a simplified version of that same chart. All we really care about is the number of days a certain balance is in effect and the balance for that time period<table border = "1" cellpadding = "5"><tr><td>Number of days</td><td>Balance</td></tr><tr><td>2</td><td>120</td></tr><tr><td>4</td><td>170</td></tr><tr><td>3</td><td>95</td></tr><tr><td>6</td><td>124.59</td></tr><tr><td>9</td><td>94.59</td></tr><tr><td>7</td><td>216.19</td></tr></table>


A = number of days
B = balance
Form a new column where we multiply A and B
Example: A*B = 2*120 = 240 for the first item in this third column<table border = "1" cellpadding = "5"><tr><td>A = Number of days</td><td>B = Balance</td><td>A*B</td></tr><tr><td>2</td><td>120</td><td>240</td></tr><tr><td>4</td><td>170</td><td>680</td></tr><tr><td>3</td><td>95</td><td>285</td></tr><tr><td>6</td><td>124.59</td><td>747.54</td></tr><tr><td>9</td><td>94.59</td><td>851.31</td></tr><tr><td>7</td><td>216.19</td><td>1513.33</td></tr></table>
If you were to add up everything up in the A*B column, then you should get 4317.18


Divide that over the 31 days to get an average daily balance (ADB) of 4317.18/31 = 139.263870967742 = 139.26



Answer: <font color=red>$139.26</font>
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