Question: An AI model evaluates soil health using 4 red soil sensors, 5 green moisture sensors, and 1 blue nutrient sensor. If one sensor is activated each day over 10 days, and sensors of the same color are indistinguishable, how many unique activation sequences are possible? - 500apps

Understanding the Context

In this SEO-optimized article, we explore the combinatorial math behind this sensor scheduling problem and reveal how AI-driven analysis helps farmers maximize data accuracy while minimizing operational complexity.

Understanding the Sensor Setup
The soil monitoring system consists of:
- 4 Red Soil pH sensors (sample type: chemical acidity)
- 5 Green Moisture sensors (sample type: water content)
- 1 Blue Nutrient sensor (monitoring key elements like nitrogen, phosphorus, potassium)

All sensors are functionally distinguishable only by color — Red, Green, and Blue — but within each color category, they are completely indistinguishable. Over 10 days, one sensor activates per day, meaning all 10 activations are used exactly, with repetitions allowed only in color matching, not individual sensor identity.

Key Insights

The Combinatorial Challenge
We want to count how many unique sequences of 10 sensor activations can be formed, given the limited number per color:
- Up to 4 Red sensors
- Up to 5 Green sensors
- Exactly 1 Blue sensor (used exactly once per logic model)

Since sensors of the same color are indistinguishable, we’re dealing with a multiset permutation problem under capacity constraints.

However, note: the Blue sensor must activate exactly once in the 10-day period, and the rest 9 activations come from Red and Green (with repetition allowed up to available limits).

Let’s define the sequence:
- Fixed: 1 Blue sensor appears once
- Remaining: 9 activations filled by Red and Green sensors
Let \( r \) be the number of Red sensors used (0 ≤ \( r \) ≤ 4)
Let \( g = 9 - r \) be the number of Green sensors used (must satisfy \( g ≤ 5 \))

Final Thoughts

So valid values of \( r \) satisfy:
- \( 0 \leq r \leq 4 \)
- \( 9 - r \leq 5 \ →\ r \geq 4 \)

Thus, only possible when \( r = 4 \) → then \( g = 5 \)

Only one valid split: 4 Red, 5 Green, 1 Blue sensors in the 10-day cycle.

Counting the Number of Unique Sequences
Now, how many unique arrangements are possible using 4 identical Red, 5 identical Green, and 1 distinct Blue sensor across 10 distinct days?

This is a multinomial coefficient accounting for indistinguishability within color groups: