Answer:
Last year there were 550 bison in the heard and this year it is 10% larger.
10% is also 10/100 or 0.10
To find 10% of the heard last year
550 x.0.10 = 55. To find the number in the heard this year add 550 (last year) + 55 (10% growth) = 605 or,
to find how many there are now last years heard size was 100% of the heard or 1.00. To find any percent more add it to 1.00 and multiply by the starting amount.
550 x 1.10 = 605
For the second example.
If there are 550 bison in the heard this year, and this year was a decrease of 10% from last year we can express last year's number like this.
550 = (x - (x * 0.10)) simplify;
550 = 1x - 0.10x
550 = 0.90x
Divide both sides by 0.90
x = 611.11 but it's hard to have 0.11 of a bison so realistically 611 bison last year. Subtract 10% or 61 and you indeed have 550 this year.
