Write each rate as a fraction in lowest terms. 4.20 for 36 potatoes

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.