It is reasonable because if you round 0.62 to the nearest tenth you get 0.6, and if you round 2.79 to the nearest whole you get three. If you divide 3 by 0.6, you get five.
Now, if you were to do the actual problem, you would get 4.5 pounds. 4.5 when rounded is equal to 5 therefore his estimate is reasonable.
Hope this helps c:
