The answer is 72 seconds.
The distance between them at the start is 12 steps.
In one minute, the father did 80 steps, Hanna did 90 steps.
Thus, in one minute Hanna gained 10 steps on her father, so she is still 2 steps behind. This just shows you the answer is more than 60 seconds, which leaves one answer.
As for how to calculate
The formula for this is t = s / delta_v
or
t = s / (v[SUB]h[/SUB]-v[SUB]f[/SUB])
where t = time
s = distance (number of steps)
v[SUB]h[/SUB] = velocity of Hanna (in steps per second)
v[SUB]f[/SUB] = velocity of father (in steps per second)
We want the time. s = 12 steps, v[SUB]h[/SUB] = 1.5 steps/second, v[SUB]f[/SUB] = 1.333 steps/second
so it is t = 12 / (1.5 - 1.3333)
so t = 72s