Calculate the number of ceiling plywood sheets required and the 3% wastage of timber. The joists are 5 cm by 400 cm, and the beam is 20 cm by 400 cm.

First, calculate the total area of the ceiling:

Ceiling area = length x width
Ceiling area = 400 cm x 400 cm
Ceiling area = 160,000 cm²

Next, calculate the total area covered by one sheet of plywood:

Plywood sheet area = 122 cm x 244 cm
Plywood sheet area = 29,768 cm²

Now, calculate the number of plywood sheets required:

Number of plywood sheets = Ceiling area / Plywood sheet area
Number of plywood sheets = 160,000 cm² / 29,768 cm²
Number of plywood sheets ≈ 5.37

Since you cannot purchase a fraction of a plywood sheet, you will need to round up to the nearest whole number. Therefore, you will need 6 plywood sheets.

Next, calculate the 3% wastage of timber:

Wastage = 3% of total timber
Wastage = 3% of (joists + beam)
Wastage = 3% of (5 cm x 400 cm + 20 cm x 400 cm)
Wastage = 3% of (2000 cm² + 8000 cm²)
Wastage = 3% of 10,000 cm²
Wastage = 300 cm²

Therefore, you will need to account for 300 cm² of wastage when purchasing timber for the ceiling.

The provided information does not include any dimensions or quantities of the ceiling plywood sheets, joists, or beams, making it impossible to calculate the number of ceiling plywood sheets required and the 3% wastage of timber.