I just needed help with this

The Real Cat King, Gustavo, has decided to implement a food delivery system for his cats that weighs out and delivers 7 kg of cat food per day. He's looking into the accuracy/calibration of different delivery systems to determine which is best for him. He wishes to ensure that 90% of the time the food given to the cats is within 0.3 kg of the desired serving. What standard deviation will achieve this?