Two independent measurements are made of the lifetime of a charmed strange meson. Each measurement...

Step 1
It is given that the population standard deviation is $7\times {10}^{-15}$.
The lifetime of the meson is estimated by averaging the two measurements.
The standard deviation of the two measurements is obtained by dividing the population standard deviation by square $\sqrt[2]{}$.
Step 2
Thus, the standard deviation of the estimate is, $SD=\frac{7\times {10}^{-15}}{\sqrt{2}}=4.9497\times {10}^{-15}$.