Which Roman numerals cannot be repeated?

How many three-digit numbers have $9$ in them?

Write the following in Roman numerals:

The following in Roman numerals: $82$ is $LXXXII$

write $\left(\mathrm{Yes} / \mathrm{No}\right)$

Write the following in Roman numerals:

Convert $15 \mathrm{km} 215 \mathrm{m} 61 \mathrm{cm} 8 \mathrm{mm}$ into millimetres. Write it in words in the Indian system of numeration.

Akash, Vishu, Sohini and Preeti are classmates. Akash is taller than Vishu while Vishu is taller than Sohini. Preeti, who is taller than Vishu and Sohini, is shorter than Akash. Find out who among them is the tallest and the shortest. Also write their names from the tallest to the shortest.