The average temperature for Monday, Wednesday and Friday was $41^\circ$C. The average for Wednesday, Friday and Thursday was $42^\circ$C. If the temperature on Thursday was $43^\circ$C, then the temperature on Monday was:

40°

Sum of temp. of monday, wednesday and friday = 41 x 3 = 123° C

Sum of temp. of wednesday, friday and thursday = 42 x 3 = 126° C

Temp. on thursday = 43° C

Temp. on wednesday and friday = 126 - 43 = 83° C

Temp. on monday = 123 - 83 = 40° C