A barista has at most 900 ounces of brewed coffee available and exactly 1 hour (3,600 seconds) to prepare drinks during the morning rush. Each small cup requires 9 ounces of coffee and 30 seconds to prepare, and each large cup requires 15 ounces of coffee and 42 seconds to prepare. The barista prepares the drinks one at a time and sells only small and large cups.
If s represents the number of small cups and ℓ represents the number of large cups sold, what is the greatest possible value of ℓ such that neither the coffee supply nor the time limit is exceeded?