If a quantity B is a part of another quantity A, then A can be written as the sum of B and some third quanrity C.

Explanation

To solve the question, we need to analyze the statement: "If a quantity B is a part of another quantity A, then A can be written as the sum of B and some third quantity C."

### Step-by-Step Solution:

1. **Understanding the Relationship**:
- Let's denote the quantities as follows: A is the whole quantity, B is a part of A, and C is another quantity that, when added to B, gives us A.

2. **Expressing the Relationship Mathematically**:
- Since B is a part of A, we can express this relationship as:
\[ B < A \]
- This means that A is greater than B.

3. **Introducing the Third Quantity C**:
- To express A in terms of B and C, we can write:
\[ A = B + C \]
- Here, C represents the remaining part of A that, when added to B, will equal A.

4. **Justifying the Statement**:
- According to Euclid's axioms, specifically the axiom that states "the whole is greater than the part," we can conclude that if B is a part of A, then there must exist some quantity C such that when added to B, it results in A.
- Therefore, the statement "If a quantity B is a part of another quantity A, then A can be written as the sum of B and some third quantity C" is indeed true.

### Conclusion:
Thus, the statement is **True**.