Respuesta :

the sum of 7+10 is 17 
7+10 = Solution 

Lets suppose there are 7 ball. After half an hour you add 10 ball to the pile. Now we have 7+10 = 17 balls in total. 

0000000 = 7 balls 

+ 0000000000 = 10 balls 

= 00000000000000000 = 17 balls total