Problem: You have over-written the function `mean()` with a variable called `mean` (which is a float), and these cannot be called as a function.¶

Solution: 1: Use a different name for our mean¶

from numpy import mean

a = [1.0, 2.0, 3.0, 2.0, 1.0]
meanval = sum(a)/len(a)
print(meanval)

b = [2.0, 5.0, 7.0]

print(mean(b))

1.8
4.666666666666667

Solution: 2: import numpy as np and use `np.mean`¶

import numpy as np

a = [1.0, 2.0, 3.0, 2.0, 1.0]
mean = sum(a)/len(a)
print(mean)

b = [2.0, 5.0, 7.0]

print(np.mean(b))

1.8
4.666666666666667