If the distance between two directrices of a rectangular hyperbola is 15, then the distance between its foci in units is:

We know that the eccentricity of a rectangular hyperbola is e=2 Distance between the directricess =2ae ,where 2a is the length of the transverse axis.So 2ae=15⇒2a=152 Distance between the foci =2ae=152×2=30.