Write the percentage uncertainty as a decimal uncertainty. Divide the percentage uncertainty by 100 to obtain the decimal uncertainty. For example, 15 percent uncertainty can be written as a decimal uncertainty of 0.15 = 15/100 = (percentage uncertainty)/100.
Multiply the decimal uncertainty by the velocity to obtain the uncertainty in velocity. For example, if the velocity is 200 m/s and the decimal uncertainty is 0.15, the uncertainty in the velocity is 30 = 0.15 x 200 = (decimal uncertainty) x (velocity).
Write the plus-minus sign before the uncertainty of velocity value. For example, an uncertainty of 15 percent in the velocity of 200 m/s is written as +/- 30 m/s. This indicates that the range of uncertainty is twice as large as the value of the uncertainty. For example, an uncertainty of 30 m/s shows that the velocity could be up to 30 m/s more or less than the given value for the velocity.