Calculating added percentages

Hello everyone, I imagine this has been queried before but I've not been able to find a clear understandable (to maths dummy me) answer, if someone could help I'd be grateful.

I have to invoice a fee to a Japanese client which includes 20% tax which the Japanese government deducts at source. i.e. 20% will be deducted from the total invoice amount, not the fee I'm to receive.

eg. say my fee is 10,000 yen. If I calculate 20% of that and invoice 12,000 yen the total value of the invoice has increased. The tax office would deduct 20% from 12,000 yen, so I'd only receive 9,600 yen.

So how do I calculate the correct percentage?

Many thanks!