You are talking about "sixty-year-old".
The Heavenly Stems (ten stems) are: A, B, C, D, Wu, Ji, Geng, Xin, Ren, and Gui.
The earthly branches (twelve branches) are: Zi, Chou, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu and Hai.
Write the Heavenly Stems on the upper line and the Earthly Branches on the lower line. Match the corresponding characters above and below to form "Jiazi, Yichou, Bingyin, Dingmao, Wuchennian, Ji" respectively. Si, Gengwu, Xinwei, Renshen, Guiyou". At this time, the heavenly stems have been used up, and there are only two characters left for the earthly branches.
How to arrange it next? When the Tiangan is used up, write it again, so the next step is "Jiaxu, Yihai". When the earthly branches are used up again, the earthly branches are written again, so next there are "Bingzi, Dingchou, Wuyin, Jimao,..."
In this way, write the heavenly stems and earthly branches six times. Write it five times, and the last "Jiayin, Yimao, Bingchen, Dingsi, Wuwu, Jiwei, Gengshen, Xinyou, Renxu, Guihai" just completes a "Huajiazi".
With this combination method, the Heavenly Stems in the odd-numbered positions can only be combined with the Earthly Branches in the odd-numbered positions, and the Heavenly Stems in the even-numbered positions can only be combined with the Earthly Branches in the even-numbered positions, so the Heavenly Stems in the odd-numbered positions are "A". It is impossible for "Bing, Wu, Geng and Ren" to meet the even-numbered earthly branch "Chou", so it will not be combined into "Jia Chou, Bing Chou, Wu Chou, Geng Chou, Ren Chou".
The combination of heavenly stems and earthly branches - correspondence between upper and lower parts