chemistry
posted by monique n .
magnesium has 3 isotopes. Magnesium24 has a percent abundance of 78.99%. Magnesium26 has a percent of 11.01%. what is the percent abundance of magnesium25? assume that there are no other magnesium isotopes.
how do i do this please help

Don't all of the percentages add up to 100%? And you have two of the three?

thank you

There is a fairly simple answer to this:
The question states that you assume there are three magnesium isotopes. If Magnesium 24 has an abundance of 78.99%, and Magnesium 26 has a percent abundance of 11.01%, the percent abundance of Magnesium 25 is simple to calculate.
The abundances must add up to 100%, and seeing as we are assuming that there is only one more isotope of Magnesium, we just subtract the sum of the first two abundances from 100%, as shown below:
100%  (78.99% + 11.01%)
100%  90%
10%
So the abundance of Magnesium 25 would be 10%.