The reaction time will be equal to the time the ruler takes to fall by 6.4 cm .
the ruler is falling with acceleration due to gravity which is equal to
9.8 m /s²
S = ut + 1/2gt²
s is distance of fall , u is initial velocity , t is time and g is acceleration due to gravity .
6.4 = 0 + 1/2 x 9.8 t²
t = 1.14 s .
So reaction time is 1.14 s .