Question 550154
direct proportion's formula is y=kx. In this case y=V and x=length squared
{{{V=k*l^2}}}

Let's say the length was 10 and k was 1
{{{V=1*(l0)^2}}}
{{{v=1*100}}}
vs 
{{{v=1*(20)^2}}}
v=1*400

400 is a 300% increase using the formula new minus old over old or (400-100)/100=3=300%

Now let's do the general case. Percent increase is new minus old over old

{{{V=k*l^2}}}

vs
{{{V=k*(2l)^2}}}
{{{V=k*4(l^2)}}}
new minus old over old:
{{{(k*4(l^2)-k*(l^2))/(k*(l^2))}}}

{{{k*3(l^2)/(k*(l^2))}}}
The k and l^2 cancel out and we are left with 3 which is equal to 300%