5biscuits cost 30p altogether how much do 7 biscuits cost?

First we want to find how much an individual biscuit costs. 
If 5 biscuits costs 30p, then  
5 x 1 biscuit = 30p 
divide both sides by 5
1 biscuit = 6p 

If one biscuit costs 6p, and we want to know how much 7 biscuits cost, then:

7 x  6p =42p

7 biscuits costs 42 p

I’m vary good with science if you need with that