from bisect import bisect
from itertools import accumulate


def fracKnapsack(vl, wt, W, n):
 """
 >>> fracKnapsack([60, 100, 120], [10, 20, 30], 50, 3)
 240.0
 """

 r = list(sorted(zip(vl, wt), key=lambda x: x[0] / x[1], reverse=True))
 vl, wt = [i[0] for i in r], [i[1] for i in r]
 acc = list(accumulate(wt))
 k = bisect(acc, W)
 return (
 0
 if k == 0
 else sum(vl[:k]) + (W - acc[k - 1]) * (vl[k]) / (wt[k])
 if k != n
 else sum(vl[:k])
 )


if __name__ == "__main__":
 import doctest

 doctest.testmod()