Explanation:
A) A school hall has 456 seats.
If you round is as per rules, you will round it up to get 460.
Now, if the number of tickets is 460 and all tickets are sold out, what problem will you have?
B) A bridge can safely carry a weight of 15.75 tonnes.
Same as above, rounding up will make the sign with 16 tonnes, but the bridge is not designed to handle this weight!
In both cases above you would round the numbers down to be in safer place.
450 seats or 15 tonnes are better numbers.