My 10 yr 4th grader is autistic, his math teacher does not him, i did not learn anything about benchmark fractions when comparing fractions when comparing fractions, so what benchmark fraction would i use to compare 7/10, and 5/12. how do i explain how i found the answer

Step-by-step explanation:

The key word here is "compare."  We want to compare 7/10 and 5/12.

When you see two or more fractions in a group, and are required to compare them, or to add or subtract them, think "lowest common denominator."

The "LCD" here is 60.  This is the smallest denominator evenly divisible by both 10 and 12.  

Noting that 10 times 6 is 60 (the LCD), multiply 6 times 7, obtaining 42 as the numerator of the fraction with LCD 60 equivalent to 7/10:  42/60.

Noting that 5 times 12 is 60 (the LCD), multiply the numerator 5 by 5, obtaining the equivalent fraction 25/60.

Now we're in a much better position to comepare 7/10 and 5/12:

Compare the equivalent fractions 42/60 and 25/60.  Very obviously, 42/60 is greater than 25/60.

Caution:  I did not refer to "benchmark fractions" in this argument.  However, the concept applies here:  To compare two fractions with different denominators, you must modify the fractions first so that both have the same denominator.  Look up "benchmark franctions" for more detail on this.