You can put this solution on YOUR website!
If the problem is:
then, since no calculator I know "knows" base 5 logrithms, we need to convert this logarithm to a base our calculator does "know", like base 10 or base e (ln). There is a base conversion formula for this:
Using this to convert the base 5 logarithm into a base 10 logarithm we get:
Now you can use your calculators to find the two logarithms, divide them and then subtract that from 1. (Be sure to use this order. Do not divide 21 by 5 first and then find the logarithm. That will not give you the correct answer.)