Step-by-step explanation:
1minute=60seconds
500 meters=60
100 meters=x
cross multiply
500×x=100×60
500x=6000
x=12
Mr. A will use 12seconds
450meters=70
100=a
cross multiply
450×a=100×70
450a=7000
divide both sides by 450
400=45
100=n
cross multiply
400×n=100×45
400n=4500
divide both sides by 400
divide and get your answer