Or search by topic
So the missing measurement can be found by subtracting the five known wingspans from the total: $78 - (13+16+12+10+15) = 12$ cm.
Next, we can find student B's mean using the total mean:
There are $10$ bats altogether, and the average wingspan is $13.4$ cm, so the total wingspan is $10\times13.4 = 134$ cm.
We already know that student A's bats' had a total wingspan of $78$ cm. So student B's bats' total wingspan must be $134 - 78 = 56$ cm.
So B's missing measurement can be found by subtracting the three known wingspans from the total: $56 - (13+16+13) = 14$ cm.