Final answer:
Without a current exchange rate, we cannot calculate the exact amount of US dollars Mitchell has remaining after converting from Canadian dollars.
Step-by-step explanation:
To find out how many US dollars Mitchell has remaining, we need to know the exchange rate between the US and Canadian dollars. Since there is no current exchange rate provided in the question, we cannot calculate the exact amount. However, if we had a given exchange rate, the process would involve first converting the 150 US dollars to Canadian dollars using the exchange rate, then subtracting the 20 Canadian dollars spent, and finally converting the remaining Canadian dollars back to US dollars using the same or updated exchange rate if it has changed in the meantime.