a rectangular piece of wood measuring 20cm by5cm by3cm weighs 900kg.find the greatest pressure the piece of wood can exert if placed on a table

Answer 1

That little piece of wood does NOT weigh 900 kg (maybe your car does)
I think you mean 0.900 kg or 900 grams

make its contact with the table as small as possible while not tipping over (standing on end)
so area touching table is
.05 * .03 = .0015 m^2

so
p = force/area = 0.9/.0015 = 600 Pascals

Answer 2

need weight/area, not mass /area
600 * 9.81 = 5886 Pascals