No. of choc chip cookies → 60% of 300
= 60/100 x 300
= 180

No. of lychee cookies → 300 – 180
= 120

After some choc chip cookies were sold, they became 40% of the remaining cookies. This means that the lychee cookies make up the remaining 60%. Since no lychee cookies were sold,

60% of remaining cookies = 120
10% of remaining cookies = 120 ÷ 6
= 20
100% of remaining cookies = 20 x 10
= 200

Since we know that there were 200 cookies left, this means that she sold (300 – 200 = 100) choc chip cookies.

Thanks so much for the explanation