4. the density and strength of concrete are determined by the ratio of cement and aggregate. suppose that a contractor has 480 cubic feet of a dry concrete mixture that is 70% sand by volume. how much pure sand must be added to form a new mixture that is 75% sand by volume?

The answer is 96ft^3

Explanation:

T=initial concrete mixture

S1= initial amount of sand

S2=final amount of sand

T2=final amount concrete mixture

C=amount of cement

Data:

T=480Ft^3

 the quantities of the initial mixture are

S=0.7*480=336Ft^3

C=0.3*480=144 Ft^3

 we pose the following equations

144+S2=T2

T2*0.75=S2  

if we solve

S2=144/(1/0.75-1)=432ft^3

to know how much we should add we subtract it from the initial amount of sand

Answer=432ft^3-336ft^3=96ft^3

d) the number of neutrons plus the number of protons

the answer is a)

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