If spending $3 on one unit of labor gives you 15 units, then that gives you 5 units per $1 of investment in labor, or $0.20 per unit. Likewise, spending $10 per unit of capital gives you 40 units, then that gives you 4 units per $1 of investment in capital, or $0.25 per unit. Ignoring the time investment for each, the least-cost combination of labor and capital means he needs to maximize his employment of labor and minimize his employment of capital.