下述代码可得到[95.0, 83.0, 66.0, 74.5, 77.5]，请问横线处缺失的代码是什么？

a = [100,90,80,85,75]#初试

b = [25,20,10,15,25]#复试

def f(x,y):

    _____________

#f = lambda x : x + 1

print(list(map(f, a, b)))