How Many Permutations for Pressing 4 Out of 8 Switches?

[novolts]

Good evening to all
I wonder if anyone can help please I have 8 switches b1,b2,b3,b4,b5,b6,b7,b8 I am trying to work out how many permutations I would have if I pressed any 4 of the 8 switches ie if I pressed b1,b3,b,7,b8 to make a circuit or it could be b3,b5,b6,b7 doing it by long hand I reckon it is 40 combination.
I fully understand if no one wishes to answer the question but I am completely useless at maths
Thank you to all

 

[robert2734]

8!/(4!4!)=70

 

[novolts]

Thankyou Robert will have to get paper and pencil out again