請問電鍍20µ, 即等如幾多c 或者mm 呢?

µ 應該指 µm, 即micron, 是一百萬份之一米

1 m = 1000 mm,
1 µm = 1000 mm /1000 000 = 1 / 1000 mm = 0.001 mm

20 µ = 20 x 0.001 mm = 0.02 mm.