Question 1045624
<i>...24 square foot sheets of plasterboard. Allowing 15% for Waste,...</i>


The indication is that the plasterboard item is big.  Not 24 of 1 square foot sheets; but instead, some number of 24-square foot sheets.


NEED TO COVER 438 SQR.YRDS.  How many plasterboard sheets minimum?
{{{438*yard*yard*3*3*(foot/yard)(foot/yard)}}}
{{{438*9*sqft}}}
{{{3942*sqft}}}
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The amount of plaster board used must include 15% wasted plaster board.


ALLOWING 15% WASTE
(This seems to be how you want most of your help.)
{{{100-15=85}}}
let b stand for how much surface area of the plasterboard actually used.
{{{0.85*b=3942}}}----accounts for the amount of non-wasted board.
{{{b=3942/0.85}}}
{{{b=4637.647*sqft}}}-----reasonably enough near to 4637 square feet


HOW MANY OF THE PLASTERBOARD SHEETS?
24 square feet per 1 sheet.
{{{4637/24}}}
{{{highlight(193*sheets)}}}


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Your work and results are good.  The only real trouble is the excessive accuracy.  Cut to three significant figures.  You could even cut it to just two significant figures and maybe say  190 sheets.  The 193 may be better.