f3

Prisha has not kept accurate accounting records during the financial year. She had opening inventory of $6,700 and purchased goods costing $84,000 during the year. At the year end she had $5,400 left in inventory. All sales are made at a mark up on cost of 20%.

What is Prisha’s gross profit for the year?
I could not understand the answer of this question

my understanding is : gross profit is sales -cost of sales (what has been taken from inventory to be sold ) in this case it is (84000+6700)-5400=85300
sales =85300*100/20=426500
gros profit =426500-85300=341200
why it 20%85300 ?
thank you