Using percentages
4 bags 100% (total oats and wheat bran)
1 bag 25%
20% of 1 bag 20% of 25% (amount of wheat bran) = 5%
Using decimals
There are 3.8 bags of oats and 0.2 bags of wheat bran
0.2 bags out of 4 bags is the proportion that is wheat bran
0.2 $\div$ 4 $\times$ 100 = 5%