The school store opened on the first day of school with 35 notebooks and 15 pencils. within 2 days it sold all of those items. on the first day, twice as many notebooks were sold as pencils. on the second day, for every 5 notebooks sold, 2 pencils were sold. how many notebooks and how many pencils were sold on each day

You are standing next to a really big circular lake. you want to measure the diameter of the lake, but you don't want to have to swim across with a measuring tape! you decide to walk around the perimeter of the lake and measure its circumference, and find that it's 400\pi\text{ m}400? m. what is the diameter dd of the lake?