MaxSpeedUp is also a SpeedUp so let us denote it as NewSpeedUp as for now.
=12/0.7=17.1 .......... (1)
For ideal case we know MaxSpeedUp=K .............(2)
If we take K=17 then result (1) violates result (2) as 17.1>17 i.e NewSpeedUp >K , which cannot happen . The speedup anyhow should be less than or equal to K.
If we take K=18,
Then 17.1<18 , does not violate the condition that NewSpeedUp<K.
You can think it in another way.
Calculate the efficiency with SpeedUp=17.1
Efficiency =17.1/17 = 1.005
ie Efficiency>100% , which is not possible.