When you intend to pay zakah i.e. after passing the year then go to gold market and ask the price of 120 gram gold and 200 gram silver and pay fortieth part of their total cost as zakah. Likewise, give the fortieth part of cash as well in zakah. It is the zakah of whole wealth.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband