As a design engineer, you are asked to design a lighting system consisting of a 70-W power supply and two light bulbs as shown in Fig. 2.124. You must select the two bulbs from the following three available bulbs.

R₁ = 80 Ω, cost = $0.60 (standard size)
R₂ = 90 Ω, cost = $0.90 (standard size)
R₃ = 100 Ω, cost = $0.75 (nonstandard size)
The system should be designed for minimum cost such that I lies within the range I = 1.2 A ± 5 percent.

70 W 1O R, Power Ry Supply