如何在Python中修复减法运算

Question

我正在尝试一个简单的减法运算：water_to_add应为dna_to_add减去final_volume为60.0。但是，dna_to_add和water_to_add的结果相同。为什么我对water_to_add的“减号”操作不起作用？

顺便说一句，dna_to_add的计算正确。

concentration_ng_ul = input("Enter Concentration (ng/ul): ")
size = input("Enter Size (bp): ")
c= float(concentration_ng_ul)
s = float(size)

c_ug_ul = c/1000.0

pmol_DNA_ul = c_ug_ul*1515.15*1/s

fmol_DNA_ul = pmol_DNA_ul/1000.0
fmol_DNA_goal = 20.0
final_volume = 60.0

dna_to_add = (fmol_DNA_goal)*(final_volume/fmol_DNA_ul)
print('DNA_ul',dna_to_add)

water_to_add = (dna_to_add)-(final_volume)
print('Water_ul',water_to_add)