>>> s = 'if the baseline chance of delay is 10%, engineering works add 25%, strikes add 35%, and bad weather adds 20%'
    >>> pb = 0.1
    >>> pe = 0.25
    >>> ps = 0.35
    >>> pw = 0.2
    >>> p = 1 - (1 - pb)*(1 - pe)*(1 - ps)*(1 - pw)
    >>> p
    0.649