To find the difference between the numbers, she must add the values of the spaces between 3/10 and 1, and between 1 and 1 3/5
we can see this in the image:
The mistake she made is that the first distance is 7/10, not 3/10, and thus, the distance is the sum:

The highlight of the mistake is:
which gives us the result:

Answer: 13/10