let x = cows last year
let y = bulls last year
x + y = last years total
0.9x + 1.5y = 660 (this year's total)
1.1(x+y) = 660 = 1.1x + 1.1y = 660
so:
0.9x + 1.5y = 1.1x + 1.1y
subtract 0.9x from each side:
1.5y = 0.2x + 1.1y
subtract 1.1y from each side
0.4y = 0.2x
to make x 1 multiply both sides by 5
2y = x
substitute for x in one of the original problems:
since they had 660 this year and that number is 10%
higher than last year, then we had 600 total last year.
So:
600 = x + y
substitute into that one with the 2y = x
600 = 2y + y
600 = 3y
200 = y
then 600 - y = x
so 600 - 200 = x
400 = x
So you had 400 cows and 200 bulls last year