Statistics problem, help!?

A long series of measurements on a checkweight averages out to 253 micrograms above 10 grams, and the SD is 7 micrograms. The Gauss model is believed to apply, with negligible bias. At this point, the balance has to be rebuilt; this may introduce bias, as well as changing the SD of the error box. Ten measurements on the checkweight, using the rebuilt scale, show an average of 245 micrograms above ten grams, and the SD is 9 micrograms. Has downward bias been introduced? Or is this chance variation? (You may ume that the errorbox follows the normal curve.) The null hypothesis is that the bias is 0 and the (one sided) alternative is that the bias is less than 0. Be sure to give the test statistic, the P value of the test, as well as state your conclusion.