方法一：
因为$b = 30a，$$a = 2，$所以$b = 30×2 = 60$
因为$c = 10b，$$b = 60，$所以$c = 10×60 = 600$
因为$d = 10c，$$c = 600，$所以$d = 10×600 = 6000$
则$a + b + c + d = 2 + 60 + 600 + 6000 = 6662$
方法二：
因为$b = 30a，$$c = 10b = 10×30a = 300a，$$d = 10c = 10×300a = 3000a$
所以$a + b + c + d = a + 30a + 300a + 3000a = 3331a$
当$a = 2$时，$3331a = 3331×2 = 6662$
答：$a + b + c + d$的值是$6662。$
