Write the following numbers in order from least to greatest:
\[
8.1 \times 102,9 \times 10^{3}, 2.7 \times 10^{5}
\]

Step 1 ：We are given the numbers \(8.1 \times 10^{2}\), \(9 \times 10^{3}\), and \(2.7 \times 10^{5}\) in scientific notation and asked to order them from least to greatest.

Step 2 ：To compare these numbers, we need to convert them to the same base. In this case, we can convert all numbers to the base 10.

Step 3 ：Converting the numbers to base 10, we get 810.0, 9000, and 270000.0 respectively.

Step 4 ：Comparing these numbers, we find that 810.0 is the smallest, 9000 is the next, and 270000.0 is the largest.

Step 5 ：Thus, the numbers in order from least to greatest are \(8.1 \times 10^{2}\), \(9 \times 10^{3}\), and \(2.7 \times 10^{5}\).

Step 6 ：\(\boxed{8.1 \times 10^{2} < 9 \times 10^{3} < 2.7 \times 10^{5}}\)