40 Male and 60 Female Rats.
If x died in the experiment.
30% died were male hence 70% were female.

therefore the ration is [spoiler]3:7[/spoiler]

well I don't know the real/original answer to this one. Reason I posted it.
I think the question asks the ratio of death RATE between male and female rats.

So if x is the dead no. of rats,
death rate of male rats is = 0.3x/40
And
death rate of female rats is = 0.7x/60

Hence the ratio of death rate male: female works out to be (0.3x/40)/(0.7x/60) = 9/14.

will be happy if somebody else gets the same answer.

30% died were male hence 70% were female.

we are asked about the rate so the rate has to be obtained by the percent of death female/male rats to the total female/male rats population. Once these rates are obtained they have to be compared to get the ratio... I would go with

StoneBlack

Let X be the rates died, 30*x/100 is total male rats that died. So rate of male rats death is 30x/100*40

then 70*x/100 is total female rats that died. So rat of femal rats death is 70*x/100*60

The ratio of both would earn 9:14

9/14 it is

You could draw up a nice little 2X2 matrix with male;female, dead;not dead. But its not necessary. Its simply a weighted average problem.

9:14