Australian AMC 2020 - Middle Primary Question 23

Emanuel works in a busy restaurant washing dishes. Each dirty plate from the stack on the left takes 1 minute to wash and dry, before being placed on top of the clean stack on the right. After 7 minutes, and every 7 minutes from then on, a waiter brings 4 more dirty plates and adds them to the top of the dirty stack.



How high is the stack of clean plates when the coloured plate is being washed?