Thirty percent of a radioactive substance decays in four years. By what percent does the substance decay each year?

The substance decays by 7.5% every year

Step-by-step explanation:

You can say

x+y=5
x=5-y

You know the value of x, put it into the next equation.

3(5-y)-7y=19
15-3y-7y=19
15-19 = 10y
-4=10y
-4/10=y
-2/5=y

Put y value back into either equation to find value of x.

x+y=5
x-2/5=5
x=5+2/5
x=27/5

Therefore, x,y = (27/5, -2/5)

Hope I helped :)