为什么round（15.0）== 15.0是假的？

Question

1. 输入4整数n，x[0]，x[1]，x[2] 使用n = a*x[0] + b*x[1] + c*x[2]和n<=4000 找maxi = a+b+c (a,b,c>=0)

2. 当我使用n=3164，x = [42, 430, 1309]时。答案必须是15.但为什么round(15.0)==15.0是假的？我在第17行尝试打印i, j, tf, ti, maxi时找到了它：

   x = [0, 0, 0]
   n, x[0], x[1], x[2] = map(float, raw_input().split())
   x.sort(reverse=True) #suppose x[0]>=x[1]>=x[2]
   maxi=0
   
   if x[2] == 1.0:
       print int(n)
   
   else:
       for i in range(int(n/x[0])+1):      #first loop to find a: a <= n/x[0]
           for j in range(int(n/x[1])+1):  #second loop to find b: b <= n/x[1]
               #used math to find equation I found:
               tf = float(i*(1.0-x[0]/x[2]) + j*(1.0-x[1]/x[2]) + n/x[2])
               ti = int(round(tf))
               if tf > n or tf <0:
                   break
   
               if ti==tf and ti >= maxi: #find satisfactory value
                   maxi = ti
   
               #print i, j, ti, tf, maxi
       print maxi