Question

suppose each bag costs $14.99. estimate the total cost of 5 bags

Answer 1

Round up $14.99 to $15.  Then mult. $15 by 5.  Result:  $75 (approx total cost of 5 bags).

Answer 2

Since you are estimating, I would recommend rounding the $14.99 to a $10.
$10 x 5 would be $50. So, the estimated total cost of 5 bags would be $50.