The frequency of the 15. ocm string is how many times higher than the frequency of the 30.0cm string

2 times.

Explanation:

We know that f = V / λ ... (1)

λ (a) = 15 cm

λ (b) = 30 cm

Va=Vb

Now by using the above formula;

f_{a} = Va/15 = Va = 15*f_{a}

f_{b} = Vb/30 = Vb = 30*f_{b}

As, Va = Vb

15*f_{a} = 30*f_{b}

f_{a} / f_{b} = 30/15

= 2 times