If 7 spiders make 7 webs in 7 days, then 1 spider will make 1 web in how many days?

Let the required number days be x.

Less spiders, More days (Indirect Proportion)

Less webs, Less days (Direct Proportion)

  $Spiders1:7Webs7:1⋮⋮ 7:x$ 

=> 1 * 7 * x = 7 * 1 * 7 

=> x= 7