We'll estimate the amount of energy sweets you will need for 18 days of cycling. Your friend should've estimated the amount you needed by rounding the numbers to the nearest tens place.
18 ⇒ 20
23 ⇒ 20
Now you can easily multiply 20 by 20 using mental math.
20 × 20 = 400; this is the estimated number of energy sweets you will need for 18 days of cycling.
Your friend's estimation is most likely to not be accurate because 550 sweets is far away from 400 sweets.
