Asample of wood that originally contained 100 grams of carbon-14 now contains only 25 grams of carbon-14. approximately how many years ago was this sample part of a living tree?

In order for 100 grams of Carbon 14 to decay to 25 grams, 2 half lives worth of time would have passed. 

log(100/25) / log(2) = 2 half lives 

2 * 5730 years = 11,460 years

the catch is that the top numbers are atomic masses the bottom numbers are the numbers of protons and electrons in an uncharged atom. they are the number of that atoms as you would find them on the periodic table. further, the numbers on the left are used to determine the missing numbers on the right

so here we go

atomic mass

239 + 1 = y + 134 + 3 combine like terms

240 = y + 137             subtract 137 from both terms

240 - 137 = y

103 = y

atomic number

94 = z +54 + 0

94 = z + 54               subtract 54 from both sides.

40 = z

answer