Answer:
there are 35 pennies in the per ounce of jar.
Step-by-step explanation:
Let us suppose there are x penny in per ounce in the jar.
We have been given that there are 420 pennies in a 12 ounce jar.
Therefore, we can make the below proportion

Let us solve this proportion for x which will give us the required amount of penny.
Cross multiplying, we get

Divide both sides by 12

Hence, there are 35 pennies in the per ounce of jar.