width of the flag is given as
[tex]w = 0.5 m[/tex]
speed of the car will be
[tex]v = 20 m/s[/tex]
now the time to cross the photogate by the flag is given as
[tex]t = \frac{d}{v}[/tex]
[tex]t = \frac{0.5}{20}[/tex]
[tex]t = 0.025 s[/tex]
so the time interval will be 0.025 s