\input zb-basic \input zb-ioport \iteman{io-port 05844121} \itemau{Vijayalakshmi, V.} \itemti{On multiplicity of triangles in 2-edge colouring of graphs.} \itemso{Ars Comb. 85, 341-352 (2007).} \itemab Summary: We denote by $G(n)$ the graph obtained by removing a Hamilton cycle from the complete graph $K_{n}$. In this paper, we calculate the lower bound for the minimum number of monochromatic triangles in any $2$-edge colouring of $G(n)$ using the weight method. Also, by explicit constructions, we give an upper bound for the minimum number of monochromatic triangles in $2$-edge colouring of $G(n)$ and the difference between our lower and upper bound is just two. \itemrv{~} \itemcc{} \itemut{monochromatic triangle; $2$-edge colouring} \itemli{} \end