Pls help (there is a pic)

The answer for #3 is c but i don't know why

He table below gives the atomic mass and relative abundance values for the three isotopes of element m. relative abundance (%) atomic mass (amu) 78.99 23.9850 10.00 24.9858 11.01 25.9826 what is the average atomic mass (in amu) of element m? 2.86 5.36 24.30 24.98

Which rate indicates the number of children that would be born per woman if she were to live to the end of her child bearing years