Hi,

40% of 750 is $0.04 \times 750 = 300$ so 300 of the paving stones are rectangular, 170 mm by 140 mm. The area of a rectangle is its length times its length so the area of each of these tiles is $170 \times 140 = 23,800$ square millimeters. Hence these 300 tiles will cover an area of $300 \times 23,800 = 7,140,000$ square millimeters.

How much area will the remaining 450 paving stones cover?

If you want the area in other units you can ask Google. Suppose you want the area covered by the larger paving stones in square meters, just type 7,140,000 square millimeters in square meters into the Google search window.

I hope this helps,
Penny