In Roman numerals:
6 = VI
4 = IV
9 = IX
10 = X. Hence: 6 = VI and 4 = IV are correct

Only house number A begins with 5 in the thousands place. All other house numbers begin with 2 in the thousands place.
Hence A (5247) cannot be Rohan’s house number.

The smallest 5-digit number is 10000. There are 4 zeros in 10000.

We note that all 4 numbers have the same number of digits.
To find the smallest number from the given numbers, we need to check the digit in the tens place for each number since the digits in the thousands and hundreds place are the same for all of them.
$\begin{array}{cc}\text{Number}& \text{Number of tens}\\ 28\underset{–}{0}9& 0\\ 28\underset{–}{9}9& 9\\ 28\underset{–}{5}0& 5\\ 28\underset{–}{9}0& 9\end{array}$
Since 9 > 5 > 0, the smallest number is P = 2809.
To find the largest number (which is either 2899 or 2890), we need to check the digit in the ones place.
Since 9 > 0, the largest number is Q = 2899.

The largest 3-digit number is 999.
The smallest 4-digit number is 1000.

Hence, their sum equals 999 +1000 = 1999.

To round off a number to the nearest ten, we check the last digit of the number.
If the last digit is less than 5, we round down the number to the nearest lower multiple of 10. 
If the last digit is 5 or more than 5, we round up the number to the nearest higher  multiple of 10.
In 20987, the digit in the ones place is 7. The nearest multiples of 10 for 20987 are 20980 or 20990. 
Since 7 is greater than 5, 20987 will be rounded off to 20990.

84926 = 8 ten thousands +
               4 thousands + 9 hundreds +
               2 tens + 6 ones
        
           = 80000 + 4000 + 900 + 20 + 6

To find the greatest number that can be created from the given digits, we need to arrange the digits in descending order.
In this case, we have 7 > 6 > 5 > 0.
Thus, the greatest number that can be formed from the given digits is 7650.

Given that 
One cube represents one.
Now, there are 3 cubes in thousands = 3000
There are 4 cubes in hundreds = 400
There are no cubes in tens.
There are two cubes in ones = 2
Hence, the number represented by the diagram = 3000 + 400 + 2 = 3402