Gallium has two naturally occurring isotopes. the mass of gallium-69 is 68.9256 amu and it is 60.108% abundant. the mass of gallium-71 is 70.9247 amu and it is 39.892% abundant. find the atomic mass of gallium.

Hi :)
So to find the average atomic mass of gallium you need to multiply the mass and percentage of abundance in the first isotope and then add that with the mass times percentage of abundance in the second isotope. I'll show you..
First convert 60.108% into a decimal, moving the decimal point over two places to the left. 60.108% -> 0.60108. Now you can multiply it by
0.60108 * 68.9256 = 41.4298.
So now I need to do the same thing as I did for the first isotope.
39.892% -> 0.39892.
0.39892 * 70.9247 = 28.29328

The last step is to add the two, 28.29328 + 41.4298 = 69.72308.
So the average atomic mass of gallium is 69.723.