The total number of flowers are 50. and the ratio from roses to daisies are 1: 4. how many roses does she have?

For this, the first step is to find out how parts there are.
If the flowers are 1 part roses to 4 parts daisies, there are a total of 5 parts.
If the flowers are evenly split into 5 parts, there must be 10 flowers in each part.
50/5 = 10
Roses : Daisies
    1     :      4
The bunch is 1 part roses
1 x 10 = 10 roses

20000 as first it increases by 2500 then by 5000 so next it will increase by 10000