Tom knows that, in his school. 10 out of every 85 students are left-handed. there are 391 students in tom's school. how many students in tom's school are left handed.

If your information is for a percentage out of every 85, you need the total number divided by the 85:

391 students / out of 85
= 391 / 85
= 4.6

But in every 85, (of which there are 4.6), 10 of them are left handed (not 4.6 of them)

So multiply the 4.6 by the ten left handed students:

4.6 x 10
= 46

Therefore there are 46 left handed students in Tom's school