Answer:
2541 peanut cookies were sold
Explanation:
A system of equations can be written to match the problem description. Let p and c represent the initial numbers of peanut and chocolate cookies, respectively. Then we have ...
p - c = 1820 . . . . Lyn had 1820 more peanut cookies
After selling 35% of her peanut cookies, Lyn had 1-0.35 = 0.65 of the original number left. Likewise, after selling 15% of her chocolate cookies, she had 1-0.15 = 0.85 of the original number left. Then after the sales, the difference in cookie count was 95:
0.65p -0.85c = 95
We only need to know the original number of peanut cookies (p), so we can use Cramer's rule to find the solution for p.
p = (-95 +0.85·1820)/(-0.65+0.85) = 1452/0.20 = 7260
The original number of peanut cookies Lyn had was 7260, so the number she sold is ...
0.35 · 7260 = 2541
_____
Cramer's rule says the solution to ...
ax +by =c
dx +ey =f
is given by ...
x = (ec-bf)/(ea-bd)
y = (fa-cd)/(ea-bd)