4-22. In estimating the total value of supplies on repair trucks. Baker Company draws random samples from two equal-sized strata of trucks. The mean value of the inventory stored on the larger trucks (stratum 1) was computed at $1500, with a standard deviation of $250. On the smaller trucks (stratum the mean value of inventory was computed as $500, with a standard deviation of $45. If Baker had drawn an unstratified sample from the entire population of trucks, the expected mean value of inventory per truck would be $1000, and the expected standard deviation would be:

a. Exactly $147.50

b. Greater than $250

c. less than $45

d. between $45 and $250, but not $147.50

The answer is b. greater than $250

I need help figuring out how to calculate the answer, I was using the Standard deviation of differences but I don't know if that's the formula.

thank u:)