The equation that would best represent the given scenario is,
At = (Ao) x (0.5)^(n/10)
where At is the number of candles still green in color, Ao is 1000 and n is the number of years. Substituting the known values from the given,
250 = 1000 x (0.5^n/10)
The value of n from the equation is 20. Therefore, after 20 years, only 25% of the candles are green in color.