Solve the following system of equations via elimination.
12x – 9y = 30
6x+15y=54

x=4,y=2

Step-by-step explanation:

solve one of the equations for x/y and then plug it into the other

This is a half life problem.
you'll find this page quite but i'll solve your question anyway.
time = half-life * log [bgng amt / ending amt] / log 2
time = 5,600 * log [100 / 15] / log 2
time = 5,600 * 0.82390874095 / 0.30102999566
time = 5,600 * 2.7369655942
time = 15,327 years