A common class of physics problems asks what happens to a quantity that depends on a certain parameter if that parameter changes by a given amount. Here is an approach to these problems:
Write down the formula that connects the dependent quantity
to the parameter that changes.
Write the formula twice:
Once with label 1.
Once with label 2.
Form a ratio of the two equations.
Insert the factor of change for the parameter expressed as a ratio.
Calculate the change in the dependent quantity.