If Bens starts at 4 and Juan has 0 than you add 3 and 2 and each time you do that is one day. In total it takes 4 days for them to sell the same amount of cookies with them both totaling 12.
Ben’s sales: 4+2=6, 6+2=8, 8+2=10, 10+2=12
Juan’s sales: 0+3=3, 3+3=6, 6+3=9, 9+3=12
Hope this helps!