miss lees class collected 3 more ball because 21-18 is 3. if i collect 18 balls and my friend collects 21 balls and then we add 21+18 we get 39 and then we put 39 in 13 equal groups we have 3in each group that is why they collected 3.
21-18=3
add 21+18=39
put in equal groups
divide 39 with 3 get thirteen make equal group get 3 in each group
*21-18=3
*21+18=39
*39 divided with 3=13
* 3 in each group
*Answer 3