Two hoses, A and B, are used to fill a fish tank with water. Hose A puts water into the tank twice as fast as hose B. If both hoses are used, the tank is filled five minutes faster than if just hose A is used. How many minutes would it take for hose B to fill the tank on its own?

it would take 5 mins for hose b to fill the tank on its own I think

Let $A$ be the rate that hose $A$ pumps water. From the given information, hose $B$ pumps water at a rate of $.5A$. Then, let $V$ equal the volume of the tank, and $t$ equal the time for hose $A$ to fill the tank on its own. The volume filled will be equal to the rate multiplied by the time, so we can set up two equations: / begin{align*}

V&=t (A) / /

V& = (t-5) (A+.5A)

/end{align*}The first represents the tank filled by just hose $A$, and the second represents the tank filled 5 minutes faster by both hoses. Setting the two equations equal to each other, we can solve for $t$ as shown: / begin{align*}

(t-5) (A+.5A) &=t (A) / /

/Rightarrow/qquad (t-5) (1.5A) &=tA//

/Rightarrow/qquad 1.5 (t-5) &=t//

/Rightarrow/qquad 1.5t-7.5&=t//

/Rightarrow/qquad. 5t&=7.5//

/Rightarrow/qquad t&=15

/end{align*}So, it would take 15 minutes for hose $A$ to fill the tank alone. Since hose $B$ is half as fast, it would take $/boxed{30}$ minutes for hose $B$ to fill the tank on its own.