Your question is Double-Sided Coin Probability. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
You have a bag of coins where one coin is double-sided (heads on both sides) and the rest are fair. You pick a coin at random and flip it three times; it lands on heads each time. What is the probability that you chose the double-sided coin?