Answer:
8
Step-by-step explanation:
Using the information provided we can say that the largest number of steps for either Sydney or Tom would be 8999 making the digit in the thousands place an 8. This is because both Sydney and Tom counted 4 digits meaning the lowest number for either of them is 1000 steps. Since the question also states that the combination/total of both their steps is still a 4 digit number, this means that the total must not exceed 9,999 steps. Therefore if the minimum is 1000 the maximum can only be 8,999 before going into the 5 digit number of steps.