Answer:
d = 7.5
Step-by-step explanation:
Given that,
D varies as R and S, and inversely as t. It can be written as :
[tex]D=\dfrac{kRS}{t}[/tex]
Where
k is constant of proportionality
Put D=12, R=3, S=20, and t=5 to find k. So,
[tex]k=\dfrac{Dt}{RS}\\\\k=\dfrac{12\times 5}{3\times 20}\\\\k=1[/tex]
Now put R=15, S=4, k=1 and t=8 to find D. So,
[tex]D=\dfrac{1\times 15\times 4}{8}\\\\D=7.5[/tex]
Hence, the value of d is equal to 7.5.
