If the cost price of the goods is R200 and they are sold at a mark-up of 100%, then the selling price is equal to the cost price plus the mark-up, or:
Selling price = Cost price + Mark-up
Mark-up = 100% x Cost price
= 100% x R200
= R200
So the mark-up is R200.
Selling price = Cost price + Mark-up
= R200 + R200
= R400
Therefore, the selling price of the goods is R400.