如果我有一系列双打:
[10.2, 20, 11.1, 21, 31, 12, 22.5, 32, 42, 13.6, 23, 32, 43.3, 53, 14, 24, 34, 44, 54, 64, 15.1, 25, 35, 45, 55, 65.3, 75.4, 16, 26, 17.5,]
我想得到第一个元素和最后一个元素
firstNum = 10.2
lastNum = 17.5
我该怎么做？