Here's why that is so.
I assume that you're referring to this video
Since you're only dealing with two isotopes, you can say for sure that their abundances must add up to give 100%.
Moreover, if you take the fractional abundance of one of these two isotopes to be equal to
The fractional abundances of the two isotopes must add up to give
#underbrace(""x"")_(color(blue)("abundance of first isotope")) + overbrace(1-x)^(color(green)("abundance of second isotope")) = 1#
Now, in Tyler's first problem, he sets up the equation so that the fractional abundance of
You know that the relaatie atomic mass of an element is equal to the sum of the contributions each isotope brings to the table.
He sets up the equation and solves for
#1 - x = 1- 0.760 = 0.240#
The exact same approach is true for the second problem. This time
#1 - x = 1 - 0.075 = 0.925#
Remember, fractional abundances are simply percent abundances divided by 100.
So if an isotope has an abundance of