Order the decimals from least to greatest: 8.9, 8.79, 7.98, 9.87, 7.8, 9.78.

While comparing the positive decimal numbers, compare their whole part that is left to the decimal point and then compare the decimal part that is right to the decimal point.

The decimal number with larger whole part is the greater than the decimal number with smaller whole part. If the whole part is same then the decimal number with larger decimal part will be greater.

Observe that the numbers 7.98 and 7.8 has the same whole part as 7. But the decimal part of 7.98 is greater than that of 7.8. So, 7.98 is greater than 7.8.

Similarly, in the decimal numbers 8.9 and 8.79, the whole part is same. But the decimal part of 8.9 is greater than 8.79 because 8.9 can be written as 8.90. So, 8.9 is greater than 8.79.

And finally in the decimal numbers 9.87 and 9.78, the whole part is same but the decimal part of 9.87 is greater than 9.78. So, 9.87 is greater than 9.78.

Thus, the order of decimals from least to greatest is 7.8, 7.98, 8.79, 8.9, 9.78, 9.87.