Question 601367
If the number of white owls in Illinois has decreased from 750 to 500 in 10 years, approximate the annual rate of decrease within 3 decimals.
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Easy way:
A decrease of 250 in 10 years is an average of 25 in 1 year.

If that doesn't do it for you, here's some more detail:

You want to calculate the population decrease per year.

Population decrease = 750 - 500 = 250

Years = 10

(Population decrease)/year = {{{(250_population_decrease)/(10 years)}}} = 25 decrease/year

["Per" means "divided by" so "decrease per year" means decrease 
divided by years.]

Edwin</pre>