**Hint:** In the given question, we have been given a number whose successor is to be found. This question can be easily solved if we know the meaning of the successor of a number.

**Complete step-by-step answer:**

The successor of a number is the number next to the number, or one plus that number, or in mathematical form can be represented as:

Let there be a number \[x\], then the successor of \[x\] is given by

\[x + 1\]

**Hence, the successor of \[378915\] is \[378915 + 1 = 378916\].**

**Note:** So, for solving questions of such type, we should know about the number system. Meaning of the successor should be known . Every whole number has a successor.