Obtain a measurement. First, get a measurement in tenths of a second. It doesn't matter how long or short -- it can be 1/10 or 100/10; it's all a matter of rearranging the fraction to get a whole number or a whole number followed by decimals.
For example, say your measurement is 5/10 of a second.
Take your measurement and, using a calculator or a piece of paper, divide the numerator by the denominator to give you a new number. In the example, 5/10 becomes 0.5.
Record your answer. The number you end up with is the amount of time in seconds. In the example, 5/10 of a second is half a second, or 0.5 seconds.