So let's leave the 112 aside for now. The question then becomes how to write the decimal 0.001 as a fraction. The key is to remember that
0.x corresponds to "tenths" so x/10
0.0x corresponds to "hundredths" --> x/100
0.00x corresponds to "thousandths" --> x/1000
The number 0.001 is just 1 thousandth, or 1/1000
Now back to 112.001. You can write it as
112/1 + 1/1000 =
(112,000+1)/1000 =
112,001/1000 (the fraction you were looking for)
Let me know if you have questions