Let's denote the total number of apples the fruit seller originally had as $x$.

1. Spoiled apples: 10% of the apples were spoiled during transport, so the number of spoiled apples is $0.1x$.

2. Remaining apples: After removing the spoiled apples, the remaining number of apples is $x - 0.1x = 0.9x$.

3. Sold apples: The seller then sold 90% of the remaining apples, so the number of apples sold is $0.9 \times 0.9x = 0.81x$.

4. Leftover apples: After selling, the seller was left with 81 apples. According to the problem, $0.9x - 0.81x = 81$.

Now let's solve this equation:

$0.09x = 81$

$x = \frac{81}{0.09} = 900$

So, the fruit seller originally had 900 apples.

Would you like more details or have any questions? Here are 5 related questions you might find interesting:

1. What if 20% of the apples were spoiled during transport?
2. How many apples would the seller have left if he sold 80% of the remaining instead?
3. What if the seller was left with 100 apples after selling 90% of the remaining?
4. How would the problem change if 15% of the apples were spoiled and he sold 85% of the remaining?
5. What would happen if the seller had sold 95% of the remaining apples instead?

Tip: In percentage-based problems, carefully set up equations based on the relationships between different percentages to avoid confusion.