how do i calculate atomic mass using the attachment

Question

highschoolmom2010 · Answer

attachment

gebooors · Answer

It is 0.075*6.015 + 0.925*7.016

highschoolmom2010 · Answer

i dont understand that at all :/

gebooors · Answer

There is 7.5 per cent isotope 6 and 92.5 per cent isotope 7

Atomic mass of Lithium is a mixture of these two.
It is like weighed average.

highschoolmom2010 · Answer

oh but if it is a weighted average wouldnt i have to do $$\frac{ 0.451125+6.4898 }{ 2 }$$  because with average you add all together and divide by the number of terms.

gebooors · Answer

Weighing is noted in per cent values

You can think you have 7.5 lighter nucleus and 92.5 heavier and you
Divide by 100

highschoolmom2010 · Answer

0.075*6.015 + 0.925*7.016=

 0.451125+6.4898=

6.940925


would i need to divide  that by 100?

gebooors · Answer

No, because it is done:  7.5 /100,
That how you get value 0.075.
And same with 92.5

highschoolmom2010 · Answer

oooh ok so i just do 

0.075*6.015 + 0.925*7.016=

 0.451125+6.4898=

6.940925

gebooors · Answer

That is right!