normal distribution?

The weekly sales of petrol at a garage have a normal distribution with mean 50 kilolitres and standard deviation 20 kilolitres and are assumed to be independent of each other.
(a)After an increase of petrol duty, the sales of petrol at the same garage are, on average, less than 25 kilolitres in 3 out of 7 weeks and more than 85 kilolitres in 1 out of 40 weeks. Determine the new mean and standard deviation, to the nearest kilolitre, of the weekly sales, assuming that the sales still have a normal distribution.(ans:mean=30 S.D.=28)