The sample mean is 49.67 while the population mean should be 42.5 so the sample mean is 7.17 grams above the population mean, which is 7.17/3.67=1.96, about 2 standard deviation away from the population mean. Hence, the probability to be on either side is 5% in total and the probability to be higher, or on the right hand side, is 5/2=2.5%.

I understand the first half of the explanation, but I don't understand ..."the probability to be on either side... onwards. How did they determine the 5%?