Katherine's junior takes 1/4*20 = <<1/4*20=5>>5 more hours to build a similar website as Katherine.
The total number of hours Katherine's junior takes to build a website is 20+5 = <<20+5=25>>25 hours.
If Katherine gave her junior 30 websites to build, her junior spent 30*25 = <<30*25=750>>750 hours on building the 30 websites.
#### 750