Utilization Equation question


I have been reading this book from how to brew from john palmer and i wonder if anyone out there is familiar with the equation below. More specifically, its been driving me crazy all morning trying to figure out where the 7/10 came from. Does anyone know? I have posted below the quote from the book:
For example, to calculate the utilization for a boil gravity of 1.057 at 30 minutes, look at the utilization values for 1.050 and 1.060. These are .177 and .162, respectively. There is a difference of 15 between the two, and 7/10ths of the difference is about 11, so the adjusted utilization for 1.057 would be .177  .011 = 0.166.
dont know where the 7/10 came from. thank
