From 6 positive numbers and 6 negative numbers, how many groups of 4 numbers, yielding a positive product, can be formed?  Pick one:
A) 720
B) 625
C) 30
D) 960
E) 255
When will the product of 4 numbers be positive? It is positive when the number of negative numbers in the product is even i.e. when we have 0 negative numbers or 2 negative numbers or 4 negative numbers.   3 cases:   - 0 negative numbers - All numbers are positive - Selected in 6C4 = 15 ways   - 2 negative numbers - 2 are positive and 2 are negative - Selected in 6C2*6C2 = 15*15= 
<gadget id="calculator">15*15</gadget>
<output>225</output>
225 ways   - 4 negative numbers - All numbers are negative - Selected in 6C4= 15 ways   Total number of ways = 15 + 225 + 15= 
<gadget id="calculator">15 + 225 + 15</gadget>
<output>255</output>
255   Answer (255)
<result>E</result>
From 6 positive numbers and 6 negative numbers, how many groups of 4 numbers, yielding a positive product, can be formed?