The formula for exponential decay is:
y = a(1 - b)x
y = the final amount
a = the original amount
b = the percentage change as a decimal, so 1 + b is the decay factor
x = time
The coal stock at a mine decreases by 20% each year. There are currently about 5 million tons left at the mine. How much coal will be left in 10 years' time?
a = 5000000
b = 20% = 0.20 = 0.2
x = 10
y = a(1 - b)x = 5000000 (0.8)10 = 536871
There will be roughly 537000 tons.