Estimating the abundance of type-Ia supernovae using magnitude limited survey
The rudiments of a solution would be take your apparent magnitude limit and work out how far away you can see type Ia supernovae and how far away you can see core collapse supernovae.

Within the latter distance, 30% of the supernovae would be type Ia. Beyond that distance, all of the observed supernovae would be type Ia.

You then need to be assuming something about the density of supernova progenitors and thinking about any spread in their absolute magnitudes. And you didn't mention any observational uncertainties.