1) Since they are to an accuracy of 1 significant figures, it means that the distance could be 45 m or 54 m but it will be expressed as 50 m. In the same way, 5m/s could actually be 4.5 m/s or 5.4 m/s.
Time taken is given by Distance/Speed.
Which pair of information would give the lowest time?
2) Same thing for this one. The upper and lower bounds for the crane are 5250 kg and 5340 kg and that for the crates are 5 kg and 14 kg.
From this, the number of crates that can be raised by the crane is given by (Breaking strain)/(Weight of crate)
When is this the largest?
3) Care to try out this one?