Vasco likes spare ribs, R, and fried chicken, C. His utility function is: U(C, R) = 40R^2C. His weekly income is $180, which he spends on only ribs and chicken. If he spends $20 for a slab of ribs and $10 for a chicken, what is his optimal consumption bundle? That is optimal value of ribs (R) = _____ units.