The answer is 72,000 mealworms.
Step 1. Calculate the number of mealworms per 1 square ft.
The square of compost 5 ft by 5 ft, has the area: A = 5² = 25 ft²
On 25 ft² there are 2,000 mealworms. On 1 ft² there are x mealworms.
25 ft² : 2,000 mealworms = 1 ft² : x
x = 2,000 mealworms * 1 ft² : 25 ft²
x = 80 mealworms.
Step 2. Calculate the area of the new square with side length that is six times longer.
A = (5*6)² = 30² = 900 ft²
Step 3. Calculate the number of mealworms (x) on 900 ft², if there are 80 mealworms per 1 ft².
x : 900 ft² = 80 mealworms : 1 ft²
x = 80 mealworms : 1 ft² * 900 ft²
x = 72,000 mealworms.
