If each of the dimensions of a rectangle is increased by 100%, then the area is increased by

Breadth $=y$

Area $=xy$

Since the dimensions are increased by $100\%$

New length$=2x$

New breadth $=2y$

New area$=2x\times 2y=4xy$

$\therefore \text{Increased area}=\frac{4xy-xy}{xy}\times 100\%$

$=\frac{3xy}{xy}\times 100\%$

$=300\%$