Let ω′ be the common velocity. Then from conservation of angular momentum, we have(I+2I)ω′=I(2ω)+2I(ω)⇒ω′=34​ωFrom the equation, angular impulse = change in angular momentum, for any of the disc, we haveτ⋅t∴τ​=I(2ω)−I(34​ω)=32Iω​=3t2Iω​​