Answer 
Find out the how many minutes are spent preparing.
To proof
As given
A recipe takes 90 minutes to make. 
25 percent of the time is spent preparing the ingredients.
25% is written in the decimal form

= 0.25
Time spent in preparing = 0.25 × 90
  = 22.5 minutes 
Therefore the 22.5 minutes are spent preparing the ingredients.
Hence proved