For each trade, attach the most recent usd_inr rate on or before its time (use pd.merge_asof). Store it in result.
import pandas as pd
trades = pd.DataFrame({"time": pd.to_datetime(["2025-01-02", "2025-01-05", "2025-01-09"]), "qty": [10, 5, 8]})
rates = pd.DataFrame({"time": pd.to_datetime(["2025-01-01", "2025-01-04", "2025-01-08"]), "usd_inr": [83.1, 83.4, 83.0]})Store your final answer in a variable named result.
Run your code to see the output here.