A trader mixes two types of beans, Wairimu costing Ksh. 4000 per bag and Yellow Green costing Ksh. 3500 per bag. If he has 30 bags of Wairimu and 50 bags of Yellow Green, what should be the selling pr

The selling price of one bag of the mixture is $\boxed{\text{Ksh. } 4425}$.

The ratio $a:b$ is $\boxed{67:33}$.

In part (a), the mixture was 30 bags of Wairimu and 50 bags of Yellow Green.
Total parts = $30+50 = 80$.
Fraction of Wairimu in mixture (a) = $\frac{30}{80} = \frac{3}{8}$.
Fraction of Yellow Green in mixture (a) = $\frac{50}{80} = \frac{5}{8}$.
So, one bag of mixture (a) contains $\frac{3}{8}$ units of Wairimu and $\frac{5}{8}$ units of Yellow Green.

In part (b), the ratio of Wairimu to Yellow Green is $a:b = 67:33$.
Total parts = $67+33 = 100$.
Fraction of Wairimu in mixture (b) = $\frac{67}{100}$.
Fraction of Yellow Green in mixture (b) = $\frac{33}{100}$.
So, one bag of mixture (b) contains $\frac{67}{100}$ units of Wairimu and $\frac{33}{100}$ units of Yellow Green.

Total Wairimu = (Wairimu from bag (a)) + (Wairimu from bag (b))

To add these fractions, find a common denominator, which is 200.

Total Yellow Green = (Yellow Green from bag (a)) + (Yellow Green from bag (b))

Using the common denominator 200:

Since both fractions have the same denominator, the ratio is simply the ratio of their numerators.

The ratio of Yellow Green to Wairimu in this new mixture is $\boxed{191:209}$.