Given that b= {1,2,3,4}, how many subsets have exactly two elements?

 Let's look at a set B = {1, 2, 3, 4}. 

There is one subset with no elements { } or ø. 
There are 4 subsets with 1 element: {1}, {2}. {3}, {4} 
There are 6 subsets with 2 elements: {1, 2}, {2, 3}, {3, 4}, {1, 3} ,{2, 4}, {1, 4} 
There are 4 subsets with 3 elements: {1, 2, 3}, {1, 3, 4}, {2, 3, 4}, {1, 2, 4} 
There is one subset with 4 elements, B. 

There are 16 subsets in all. 

The rule is a set with n elements has 2^n subset. hope this helps :)

Best answer that i found on yahoo ok   set :   x= length of 2nd piece in cm  x- 3= length of 1st piece in cm  x-4= length of 3rd piece in cm  then since you know they all came from the 20 cm ribbon you add them together and set them equal to 20x+ x-3 +x-4 = 20  3x - 7= 20  3x= 27  x= 9  the shortest piece is the third piece which is x- 4  x-4  9-4 = 5  the length of the smallest piece is 5 cm.  the other lengths are  9 cm  6 cm