This problem can be solved by using the formula for linear motions. The formula relating velocity, distance and time is:
t = d / v
where t is time, d is distance and v is velocity
We are given that d = 24 blocks
When v = 1 block / minute
t = 24 blocks / (1 block / minute)
t = 24 minutes
When v = 2 block / minute
t = 24 blocks / (2 block / minute)
t = 12 minutes
Hence we can see that it takes half the time when she is travelling at 2 blocks per minute.