State True/False:

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

The statement is true.

If B is part of A, then A can be divided into two parts, B and another quantity C. 

Therefore, A can be written as the sum of B and C. 

This can be represented as A = B + C.

This is in accordance with Euclid's fourth axiom which states that things which coincide with one another are equal to each other. 

In this case, A coincides with the sum of B and C, so A is equal to B + C.