Final answer:
The original cost of the mobile before the 10% discount was $10000. This was calculated by dividing the post-discount price of $9000 by 0.90, which represents the 90% final value after applying the discount.
Step-by-step explanation:
To find the original cost of the mobile before the discount, we use the information that after a 10% discount, the price is $9000. The 10% discount means the $9000 is 90% of the original price, since 100% - 10% = 90%. We can set up an equation where 90% (or 0.90) multiplied by the original price (let's call it P) is equal to $9000.
Equation: 0.90 × P = $9000
To find the original price P, we divide $9000 by 0.90:
P = $9000 / 0.90
P = $10000
Therefore, the original cost of the mobile before the discount was $10000.