How much yield with a 400 watt hps

A question from a fellow grower:

can I get up to lb with a 400 watt hps in a 3.5x 3.5. How many plant. I want info on that

We need a lot more info than that.

Nutrient system:
Indoor or Outdoor? Indoor
Size of space: 3.5 x 3.5
Soil or Hydro?
Medium used:
EC/TDS ppm levels:
Humidity %:
Light system/watts? 400

Thanks; For helping us, help you. Peace lw