calculator output of 4E-7
You entered the wrong calculation into your calculator.
You entered 400×10-9, instead of 400-9.
These are absolutely not the same thing!
The C++ program is correct: pow(400, -9) calculates 400-9, which is approximately 3.815×10-24.
Here is some further reading for you:
7
solved C++ pow(400,-9) is giving wrong answer [closed]