Find the number which would come in place of question mark 81 : 101 ::  121 : ? .

Break Down the Problem

1. We are given a proportion where 81 relates to 101 and 121 relates to an unknown value.
2. We need to find the relationship between the pairs to determine the value corresponding to the second pair.

Relevant Concepts

To find the relationship, we can express the pairs mathematically and look for a pattern.

Analysis and Detail

1. First, find the difference between the numbers in the first pair: $101 - 81 = 20$

2. Now, we can apply the same difference to the second pair. We add 20 to 121 to find the unknown: $? = 121 + 20$

3. Calculate the unknown: $? = 141$

Verify and Summarize

The relationship maintained in both pairs suggests that the same difference of 20 applies. Thus, the calculations are correct.

Final Answer

The number that would come in place of the question mark is 141.