In a camp, there is enough meal to feed either 120 men or 200 children. If 150 children have already taken the meal, how many men can be served with the remaining meal?

Introduction / Context:
This question involves equivalent consumption of meal by men and children. It tests your ability to convert between two types of consumers using a ratio and then work out how much of the resource remains. Such problems often appear in aptitude tests under chain rule or ratio and proportion topics.

Given Data / Assumptions:

• The total stock of meal is enough for 120 men.
• The same stock is also enough for 200 children.
• So the feeding capacity for men and children is equivalent.
• 150 children have already taken the meal.
• We must find how many men can be fed with the remaining meal.

Concept / Approach:
The total meal can be measured either in man meals or child meals. It is convenient to use child meals first. After some children have eaten, we determine the remaining child meal capacity and then convert that remaining capacity into an equivalent number of men using the known equivalence between men and children.

Step-by-Step Solution:
Step 1: Total meal is enough for 200 children, so think of it as 200 child meals.Step 2: 150 children have eaten, so child meals consumed = 150.Step 3: Remaining child meals = 200 - 150 = 50.Step 4: From the original equivalence, 120 men correspond to 200 children.Step 5: Therefore, 1 man corresponds to 200 / 120 = 5 / 3 children.Step 6: Inverting this, 1 child corresponds to 3 / 5 of a man.Step 7: Remaining child meals (50) correspond to 50 * (3 / 5) men = 30 men.

Verification / Alternative check:
You can also convert the total meal to man meals first. Since 120 men correspond to 200 children, each child is equivalent to 120 / 200 = 3 / 5 of a man for one meal. When 150 children eat, that equals 150 * 3 / 5 = 90 man meals consumed. The original stock was 120 man meals, so remaining man meals = 120 - 90 = 30, which confirms the earlier result.

Why Other Options Are Wrong:
20 or 25 men are too small, since more meal remains than that. 35 or 40 men are too many and would require more meal than is left after 150 children have eaten. Only 30 men correctly matches the equivalence relationship.

Common Pitfalls:

• Assuming that 150 children equal 150 men without using the given conversion ratio.
• Mixing up which way to convert, for example multiplying by 5 / 3 when you should multiply by 3 / 5.
• Forgetting that the 150 children have already eaten, and instead trying to allocate the full capacity again.

Final Answer:
The remaining meal can feed 30 men.