#### Exercise 1.2: Statistics

Given: four values, `a`, `b`, `c`, and `d`, all real numbers;
the formulas for the mean and standard deviation
mean = (a+b+c+d)/4;
standard deviation = (((a-mean)**2 + (b-mean)**2 +
(c-mean)**2 + (d-mean)**2) / 3)**0.5

Compose: a let-in statement that returns the mean and standard deviation of the given set of numbers.

let mean := (a + b + c + d) / 4.0;
a_diff := a - mean;
b_diff := b - mean;
c_diff := c - mean;
d_diff := d - mean;
a_diff_sq := a_diff * a_diff;
b_diff_sq := b_diff * b_diff;
c_diff_sq := c_diff * c_diff;
d_diff_sq := d_diff * d_diff;
diff_sq_sum := a_diff_sq + b_diff_sq +
c_diff_sq + d_diff_sq;
sigma := sqrt( diff_sq_sum / 3.0 )
in mean, sigma
end let

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