It's a really simple question you have. Considering we know that further nitration will occur on positions 3 and 5 on the benzene ring if the temperature exceeds 60 degrees. All we have to do when we are asked for the temperature is to state ANY temperature that is reasonably below 60.. not surprising some books say 55 degrees. in my exam alevel Cambridge board CIE.. i put the answer as <60C and i got it correct.