A little more: decimal_portion

[Seymore4Head]

 A little more: decimal_portion

Write a function that takes two number parameters and returns a float
that is the decimal portion of the result of dividing the first
parameter by the second. (For example, if the parameters are 5 and 2,
the result of 5/2 is 2.5, so the return value would be 0.5)

http://imgur.com/a0Csi43

def decimal_portion(a,b):
    return float((b/a)-((b//a)))

print (decimal_portion(5,2))

I get 0.4 and the answer is supposed to be 0.5.