If the first digit you are going to throw away is 0,1,2,3, or 4, then you merely
throw it away as well as any digits to the right of it regardless of their size.

However if the first digit you are going to throw away is 5,6,7,8, or 9, you
must increase the last digit you are going to keep by 1 when throwing away the
digit and all that follow it to the right.

In the case of 4278.925, the first digit we are going to throw away is the 5 on
the right end, so we must add 1 to the last digit we are going to keep, and make
it 3.  So the answer is 4278.93.

[If it had been 4278.920, 4278.921, 4278.922, 4278.923, or 4278.924, the answer
would have been 4278.92.  If it had been 4278.926, 4278.927, 4278.928, or
4278.929, the answer would have also been 4278.93, just as it is because it was
4278.925.]

Edwin