Chemistry A-level Abundance question [Help]

Abundance of 85Rb : 87 Rb = 2.5:1 = 5:2
So, relative atomic mass of Rb
= 85 x 5/7 + 87 x 2/7
= 85.571 a.m.u.

Thank you so much! That really helps!
I have one question, it may sound obvious, but why do you not divide the final value by 100?

Thank you so much! That really helps!
I have one question, it may sound obvious, but why do you not divide the final value by 100?

The value that its been multiplied by was already divided by 100 so there was no need to do it at the end, as they just used a decimal instead. So instead of doing the ratio as the percentage out of 100 each isotope is they just used the fractions. 5/7 = 0.71528 for example instead of the 71.528% you would use if it gave you it in the question. Eliminating the step of dividing it by 100.