A community workshop offers standard and premium monthly memberships. In one month, there were 480 active memberships. A standard membership costs $18, and a premium membership costs $30.
One-sixth of the standard members received a 25% discount, and all other standard members paid the listed price. One-fourth of the premium members received a 20% discount, and all other premium members paid the listed price. The workshop collected $9,900 in membership revenue that month.
How many premium memberships were active that month?