Answer: -1.5
Step-by-step explanation:
When population standard deviation is known, then the formula to find the z-test statistic for population mean is given by :-

, where n= Sample size.
= Population mean
= Sample mean
= Population standard deviation.
As per given , we have
(average infants contracted typhoid each month)

n= 4 (No. of months)
Then , the value of the appropriate test statistic will be :




Hence, the value of the test statistic = -1.5