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)

Spiders	1	:	7		:: 7 : x
Webs	7	:	1

 1 x 7 x x = 7 x 1 x 7

 x = 7.