There are 45 trees in a row. A lemon tree is 20th from the right end. What is the position (rank) of the lemon tree from the left end of the row?

Introduction / Context:
This reasoning question deals with positions counted from the left and from the right in a single row. You are told that a particular lemon tree stands at a certain position from the right end, and you must find its position from the left end. Such problems are common in aptitude tests and become easy once you know the simple relationship between left and right positions and the total number of items.

Given Data / Assumptions:

• Total number of trees in the row is 45.
• The lemon tree is 20th from the right end of the row.
• Positions are counted starting from 1 at each end.
• We assume there is exactly one lemon tree with this position in the row.
• We need to find its position counted from the left end.

Concept / Approach:
For any object in a straight line arrangement, there is a simple formula that links its position from the left and from the right. If the total number of items is N, the position from the left is L and the position from the right is R, then L + R - 1 = N. This is because the object itself is counted once in both left and right positions. Rearranging this relation lets you find one position if you know the other and the total count.

Step-by-Step Solution:
Step 1: Let N be the total number of trees, which is 45.Step 2: Let R be the position of the lemon tree from the right, which is 20.Step 3: Let L be the position of the same lemon tree from the left, which we want to find.Step 4: Use the formula L + R - 1 = N.Step 5: Substitute the known values: L + 20 - 1 = 45.Step 6: Simplify: L + 19 = 45.Step 7: Solve for L: L = 45 - 19 = 26.Step 8: Therefore the lemon tree is 26th from the left end.

Verification / Alternative check:
You can also visualise the row. If the tree is 20th from the right, that means there are 19 trees to its right. Since the total number of trees is 45, the number of trees to its left, including the lemon tree itself, must be 45 - 19 = 26. This directly gives the left position as 26. This matches the algebraic calculation and confirms the answer.

Why Other Options Are Wrong:
If the position from the left were 24, 25 or 27, the formula L + R - 1 = N would not hold. For example, if L were 24, then 24 + 20 - 1 = 43, which does not equal 45. The same mismatch occurs for 25 and 27. Only L = 26 satisfies the relationship between left and right positions given the total of 45 trees.

Common Pitfalls:
A frequent error is to forget subtracting 1 in the formula and use L + R = N, which leads to wrong results. Another mistake is to subtract the position from the total incorrectly, for example computing 45 - 20 instead of 45 - 19. Keeping the simple formula in mind and doing careful arithmetic prevents these issues.

Final Answer:
The lemon tree is 26th from the left end of the row.