-4.5 + 4.2 + 5.6 - 7.3= -2.000000000000001−4.5+4.2+5.6−7.3=−2.000000000000001
If we consider it as a combination problem, then we could calculate the how many combinations are there if we pick up three nodes out of six nodes, and then we have:

And the answer should be 20
Step-by-step explanation:
Below is an attachment containing the solution.
Answer:
1/2
Step-by-step explanation:
one side is head and other is tails