Explanation

Given data:

The seller sells 40% of his apples.
After selling, he still has 420 apples.
Concept/Formula:

The concept used to solve this problem is related to "percentage". The seller was left with 60% of his apples (100% - 40% = 60%) when he had 420 apples left.

Solution:

⇒ the 420 apples represent 60% of his original stock (since he sold 40%).

⇒ If 60% of the original stock equals 420 apples, then 1% can be calculated by dividing 420 by 60.

Hence, 1% = 420 ÷ 60 = 7 apples.

⇒ Now, to find the total original stock (100%), the amount per 1% is multiplied by 100.

Therefore, the original amount = 7 × 100 = 700 apples.

Therefore, originally, he had 700 apples.