The cost of cementing 30 m² of floor area is $810. how much will it
cost to cement 55 m² of floor area?

$1485 cost to cement 55m^2 of floor area. You should need to know how much will it cost 1m^2, so then you can solve the area you were asked for.

for this case we must apply distributive property to the following expression:

the distributive property states that:

then, when applying the property we have to multiply the -2 for each term within the parenthesis:

33/100 is what i got