Hey there! :)
List the multiples of each number and see what's common
3 = 3,6,9,12,15,18,21,24,27,30,33,36,39,42,45,48,51,54,57,60,63,66,69
4 = 4,8,12,16,20,24,28,32,36,40,44,48,52,56,60,64,68,72
5 = 5,10,15,20,25,30,35,40,45,50,55,60,65,70
6 = 6,12,18,24,30,36,42,48,54,60,66,72
10 = 10,20,30,40,50,60,70
15 = 15,30,45,60,75
Your answer is 60
Hope this helps :)