The answer will depend on the size of the patio slabs.
140
14
400 at least
The answer will vary slightly depending on assumptions: will the slabs have a space between them, and will you be cutting the slabs to fill fractions. 21 feet is equal to 14.2 of the 450 mm slabs. 19 feet is equal to 12.9 of the slabs. To complete the rectangular area, 14.2 x 12.9 = 183 slabs.
The answer will depend on the size of the patio slabs.
20 I believe
140
You will need 63 44cm x 44cm slabs to cover that area.
14
400 at least
The answer will vary slightly depending on assumptions: will the slabs have a space between them, and will you be cutting the slabs to fill fractions. 21 feet is equal to 14.2 of the 450 mm slabs. 19 feet is equal to 12.9 of the slabs. To complete the rectangular area, 14.2 x 12.9 = 183 slabs.
305.56
23.41 sq m
A square slab measuring 0.6 metres all round has an area of 0.36 square metres. This means that to cover an area of 24 square metres you will need 66 slabs and 2/3 of a slab. Covering the area may be problematic if 1/3 of a slab and 2/3 of a slab become involved, of lesser fractions, depending on the exact length and width which gave the area of 24 - and if there is a pattern to maintain.
20 ft= 6096 mm (6096 * 6096) / (450 * 450) = 183.512178 184 slabs
If most of the off-cuts are also used then you will need 315 slabs. Otherwise you will need a lot more.