A stop watch has 10 divisions graduated between the 0 and 5s marks. What is its least count?

Using the formula given below,

$\text{Least count (L.C.)} = \dfrac{\text{ Value of one div (x)}}{\text {Total no. of div on stop watch (n)}}$

Given,

Total number of divisions on stop watch = 10

we get,

$\text{L.C.} = \dfrac{\text{x}}{\text{n}} \\[0.5em] \text{L.C.} = \dfrac{5}{10} \\[0.5em] \text{L.C.} = \dfrac{1}{2} \\[0.5em] \Rightarrow \text{L.C.} = 0.5\text{s}$

Hence, least count of stop watch is 0.5s.