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

Explanation:
Let the required number days be x.

Less spiders, More days (Indirect Proportion)

Less webs, Less days (Direct Proportion)

Spiders 1:7

Webs 7:1

}⋮⋮ 7:x

=> 1 * 7 * x = 7 * 1 * 7

=> x= 7