Answer:
The value of x when y=21 is [tex]x=\frac{5}{7}[/tex]
Given :
Y varies inversely with x. when y varies inversely with x then the equation is
[tex]y=\frac{k}{x}[/tex]
We need to find the value of k using the given information
when x=3 then y=5.
Replace the values and find out k
[tex]y=\frac{k}{x}\\5=\frac{k}{3}\\k=15[/tex]
the value of k is 15
The equation becomes
[tex]y=\frac{15}{x}[/tex]
Now we find out x when y=12
[tex]y=\frac{15}{x}\\21=\frac{15}{x}\\21x=15\\x=\frac{15}{21} \\x=\frac{5}{7}[/tex]
The value of x when y=21 is [tex]x=\frac{5}{7}[/tex]
Learn more : brainly.com/question/25046198