Remember how your algebra text book had the answer to half your homework problems in the back? And remember how you came across a problem where, even with the answer in hand, you couldn’t figure out how to solve it? Well, here comes Slader.com to the rescue.The site has compiled step-by-step answers to the problems in some of the most popular high school textbooks, according to All Things D.  Most interesting to Paywall Times readers will be their subscription model, which can be as low as $1 a month and has members (a mix of students and parents) spend “point” to see full solutions.Users pay for a block of points, subscribe for daily points, or buy “gold” that can be transferred to other members.The rationale for this is that their customers are both parents with credit cards, but also students themselves, who sometimes use pre-paid debit cards, sometimes transfer points to friends, or even use PayPal, according to All Things D.I spent some time perusing their site, and the subscription models (especially the $1/month promise) seem a bit obtuse and hidden. Furthermore, the whole model seems like it may be a retention nightmare.Then again, playing into a gaming aspect of collecting points instead of focusing on a traditional subscription angle may be the best way to retain high school students. It’s going to be interesting to see what happens and how they scale their services.