volume or area

[shweta.kalra]

A wall of measurements 30m x 12m x4m was constructed with bricks of dimensions 8cm x6cm x6cm. if 80 percent of the wall consists of bricks, find the number of bricks used for the construction
my question is do we need to know the AREA OR VOLUME OF WALL ?

[Aman verma]

A wall of measurements 30m x 12m x4m was constructed with bricks of dimensions 8cm x6cm x6cm. if 80 percent of the wall consists of bricks, find the number of bricks used for the construction
my question is do we need to know the AREA OR VOLUME OF WALL ?

Hi Sweta,

Now the logic of the question requires that we compare the volume of the wall with the volume of the bricks. Since the question mentions that 80% of the wall consists of bricks we can assume that the rest of the wall consists of mortar( Cement ) and doors and windows. So we adjust the division accordingly.
If we consider the area then the answer will be susceptible to multiple solutions which the GMAT do not allow. Also if the question is about optimisation then we have to compare each dimension to find out the optimum solution. Please mention whether it is a problem solving question or a Data sufficiency question.

[Brent@GMATPrepNow]

A wall of measurements 30m x 12m x4m was constructed with bricks of dimensions 8cm x6cm x6cm. if 80 percent of the wall consists of bricks, find the number of bricks used for the construction.

Let's answer the question.

Aside: The GMAT does not assume that one can perform metric conversions with some help. So, an official GMAT question would include some extra information like: "1m = 100cm"

Volume of wall = (30m)(12m)(4m) = (3000cm)(1200cm)(400cm)
IMPORTANT: I'm not going to evaluate this just yet. You'll see why later.
80% of the volume is bricks, so bricks take up (0.8)(3000cm)(1200cm)(400cm) cubic cm of the volume.

Volume of 1 brick = (8cm)(6cm)(6cm)

So, the number of bricks = (0.8)(3000)(1200)(400)/(8)(6)(6)
Notice that 0.8/8 = 1/10 and 3000/6 = 500 and 1200/6 = 200
So, (0.8)(3000)(1200)(400)/(8)(6)(6) = (500)(200)(400)/10
= (50)(200)(400)
= 4,000,000 bricks

Cheers,
Brent