Something appears to be missing here. Did you mean $4.20 for 36 potatoes?
If so, write the fraction
$4.20
----------------
36 potatoes
and reduce it. 6 divides into both $4.20 and 36 without a remainder:
$0.70 and 6
Thus, the "rate" is 0.70 dollar for 6 potatoes, or 0.35 dollar for three.
If you wish you could rewrite this partially in words:
"Potatoes sell for $0.35/(3 potatoes)," or $0.1166 per potato, or about 12 cents per potato.
