Every 10th customer: $1 coupon
10, 20, 30, 40, 50, 60, 70, 80, 90, 100
Every 12th customer: $2 coupon
12, 24, 36, 48, 60, 72, 84, 96, 108, 120
The 60th customer will receive both the $1 coupon and the $2 coupon.
He will receive the 6th $1 coupon and the 5th $2 coupon.