Number of moles of helium $=0.3$ Number of moles of argon $=0.4$ We know that $KE = nRT$ $KE$ of helium $=0.3 \times R \times T \ldots$ (i) $KE$ of argon $=0.4 \times R \times 400 \ldots$ (ii) According to question $KE$ of helium $= KE$ of argon $0.3 \times R \times T=0.4 \times R \times 400$ $T=533\, K$