0.9 x 100 = 90
0.9 times 100 is 90. Multiplying by 100 moves the decimal point two places to the right, so 0.9 becomes 90. It is also the step that turns the decimal 0.9 into 90 percent.
0.9 × 100 = 90
The working
0.9 is nine tenths. A tenth of 100 is 10, and nine of those is 90. The decimal-point version is quicker: shift the point two places right, filling with a zero as you pass the end of the digits, so 0.9 becomes 9. and then 90.
This is the decimal-to-percent step
Most people who run this calculation are converting a decimal into a percentage, and multiplying by 100 is exactly how that is done. So 0.9 = 90%. A decimal below 1 always converts to a percentage below 100.
Two traps worth knowing
0.9% of 100 is 0.9, not 90. Percent already means “per hundred”, so a figure written with a percent sign has been divided by 100 once already. Multiplying it by 100 again undoes that and gives an answer a hundred times too big.
0.9 hours is 54 minutes, not 90. Decimal hours convert with 60, not 100. 0.9 x 60 = 54. This one bites on timesheets, where a 0.9 entry looks like it should be an hour and a half and is nowhere near it.