In an ideal turbofan engine shown in the figure below, the compressor is driven by the high pressure turbine, and the fan is driven by the low pressure turbine. The stations 0, 2, 3, 4, 4.5, and 5 refer to free-stream, compressor inlet, compressor outlet, combustor exit, high pressure turbine exit, and low pressure turbine exit, respectively, and the subscript ‘t’ refers to the total condition. Also, 𝜏𝑟 = 𝑇𝑡0 ⁄𝑇0, 𝜏𝑐 = 𝑇𝑡3 ⁄𝑇𝑡2 and 𝜏𝜆 = 𝑇𝑡4 ⁄𝑇0 . The total temperature ratio of the high pressure turbine (𝑇𝑡4.5 ⁄𝑇𝑡4 ) is given by ______.