Added README.md file for Average Selling Price

Author: sayandip20

1390-average-selling-price/README.md

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+<h2><a href="https://leetcode.com/problems/average-selling-price">Average Selling Price</a></h2> <img src='https://img.shields.io/badge/Difficulty-Easy-brightgreen' alt='Difficulty: Easy' /><hr><p>Table: <code>Prices</code></p>
+
+<pre>
++---------------+---------+
+| Column Name   | Type    |
++---------------+---------+
+| product_id    | int     |
+| start_date    | date    |
+| end_date      | date    |
+| price         | int     |
++---------------+---------+
+(product_id, start_date, end_date) is the primary key (combination of columns with unique values) for this table.
+Each row of this table indicates the price of the product_id in the period from start_date to end_date.
+For each product_id there will be no two overlapping periods. That means there will be no two intersecting periods for the same product_id.
+</pre>
+
+<p>&nbsp;</p>
+
+<p>Table: <code>UnitsSold</code></p>
+
+<pre>
++---------------+---------+
+| Column Name   | Type    |
++---------------+---------+
+| product_id    | int     |
+| purchase_date | date    |
+| units         | int     |
++---------------+---------+
+This table may contain duplicate rows.
+Each row of this table indicates the date, units, and product_id of each product sold. 
+</pre>
+
+<p>&nbsp;</p>
+
+<p>Write a solution to find the average selling price for each product. <code>average_price</code> should be <strong>rounded to 2 decimal places</strong>.</p>
+
+<p>Return the result table in <strong>any order</strong>.</p>
+
+<p>The&nbsp;result format is in the following example.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> 
+Prices table:
++------------+------------+------------+--------+
+| product_id | start_date | end_date   | price  |
++------------+------------+------------+--------+
+| 1          | 2019-02-17 | 2019-02-28 | 5      |
+| 1          | 2019-03-01 | 2019-03-22 | 20     |
+| 2          | 2019-02-01 | 2019-02-20 | 15     |
+| 2          | 2019-02-21 | 2019-03-31 | 30     |
++------------+------------+------------+--------+
+UnitsSold table:
++------------+---------------+-------+
+| product_id | purchase_date | units |
++------------+---------------+-------+
+| 1          | 2019-02-25    | 100   |
+| 1          | 2019-03-01    | 15    |
+| 2          | 2019-02-10    | 200   |
+| 2          | 2019-03-22    | 30    |
++------------+---------------+-------+
+<strong>Output:</strong> 
++------------+---------------+
+| product_id | average_price |
++------------+---------------+
+| 1          | 6.96          |
+| 2          | 16.96         |
++------------+---------------+
+<strong>Explanation:</strong> 
+Average selling price = Total Price of Product / Number of products sold.
+Average selling price for product 1 = ((100 * 5) + (15 * 20)) / 115 = 6.96
+Average selling price for product 2 = ((200 * 15) + (30 * 30)) / 230 = 16.96
+</pre>