What proportion of spores do you expect to produce a colony | Homework Helpers

The probability distribution of the number of accidents in Grand Rapids, Michigan, each day is given by a series of mutation in Neurospora results in the requirement of either trptophan(trp) or some realted molecule (AA, IGP, idole) in order for growth to occur (plus means growth):

Strain Medium
MM MM+AA MM+IGP MM+indole MM+Trp

trp8 – + + + +
trp2 – – + + +
trp3 – – – + +
trp1 – – – – +

If you cross trp8 mutant to trpI mutant(the loci are not linked), what proportion of spores do you expect to produce a colony on the minimal medium without any supplements?

The answer is 1/4… but how do we get this number???? (please show work)