To find out how many milkshakes had whipped cream on top, we can calculate 30% of the total number of milkshakes sold, which is 20.
First, convert the percentage to a decimal: \[ 30% = 0.30 \]
Now, multiply the total number of milkshakes by the decimal: \[ 0.30 \times 20 = 6 \]
Therefore, Irma's Diner sold 6 milkshakes with whipped cream on top.