How many kilograms of potatoes will each person get if 5 people share 1/3 kilogram of potatoes equally?

You can rephrase the question into this expression.
1/3 / 5
or
1/3 / 5/1
To divide a fraction, you simply flip the divider over (5/1 to 1/5) and switch to the operation to multiplication.
1/3 * 1/5
1*1=1
3*5=15
=1/15

If split into 5 different portions, every person would 1/15 of the potatoes.

Hope this helps!

10, f(3) represents f(x), so plug 3 into all the x's