if each can hold 1/8 of a bag, then 8 can hold 1 bag. if there are 15 bags, and each bag can fill 8 planters, then 15 bags can fill 15x8 planters, or 120
You have 15 bags of topsoil and each planter holds 1/8 of 1 bag of topsoil, therefore the equation for this problem is as follows: 15 x 18 = 270 planters.
I hope this helps you and I'm very sorry if it doesn't.