Hi Amelia.
Since this is a third grade question it might be that the teacher wants to know how many pennies this is. Each dollar is 100 pennies so $10.00 worth of pennies is 10 100 pennies. Thus one answer is 10 100 = 1000 pennies long.
If you are going to measure in length units then i t depends on whether the pennies are lying flat so their round edges are touching or stacked with their flat sides to one another.
This source reports that a penny is 19.05 mm in diameter and 1.45 mm in thickness. Multiply those by $10 worth of pennies (that's 10 x 100 pennies) and you have either answer.
Hope this helps,
Stephen and Penny.
