Please help me with this question, it's in the book called the The Math of Life & Death, and my math teacher assigned this question but I don't know how to solve it. My four-year-old son loves playing out in the garden. His favourite activity is digging up and inspecting creepy crawlies, especially snails. If he is patient enough, after the initial shock of being uprooted, they will emerge cautiously from the safety of their shells and start to glide over his little hands leaving viscid trails of mucus. Eventually, when he tires of them, he will discard them, somewhat callously, in the compost heap or on the woodpile behind the shed. Late last September, after a particularly busy session in which he had unearthed and disposed of five or six large specimens, he came to me as I was sawing up wood for the fire and asked ‘Daddy, how many snails is [sic] there in the garden?’ A deceptively simple question for which I had no good answer. It could have been 100 or it could have been 1000. To be quite honest, he would not have comprehended the difference. Nevertheless, his question piqued an interest in me. How could we figure this out together? We decided to conduct an experiment. The next weekend, on Saturday morning, we went out to collect snails. After ten minutes, we had a total of 23 of the gastropods. I took a sharpie from my back pocket and proceeded to place a subtle cross on the back of each one. Once they were all marked up, we tipped up the bucket and released the snails back into the garden. A week later we went back out for another round. This time, our ten-minute scavenge brought us just 18 snails. When we inspected them closely we found that three of them had the cross on their shells, while the other 15 were unblemished. This was all the information we needed to make the calculation.

i think what i need to find is how many snails are in the garden.