Why does notebook code work only after I run another cell?

A notebook kernel remembers variables created by cells you have already run. Restarting it clears that memory. Run the setup cells before the cells that depend on them. After finishing an experiment, restart the kernel and run the notebook from top to bottom to check that it works in a clear order. The example below includes the missing setup before using total.

values = [2, 4, 6]
total = sum(values)
count = len(values)
print(total / count)

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