Percent Abundance

Same protons(atomic number) and electrons (so neutral)

Have different number of neutrons so different mass numbers

Are defined as different percents of the total average atomic mass

Often are presented as isotopic notation

multiply the atomic mass given for the isotope by its' given percent abundance 

add this to the next isotopes given atomic mass times its' given percent abundance

then divide by 100.

The formula in the video converts each percent to decimal form first, this is another way to do these problems