Because you simply set it up in a proportion box, for example if you have the fraction 4/8 you put the 4 on top of the 8 and 100 next to the 8 because with percents you always use 100. then solve,
biology solved our problem such as smoking problem:first you have to ask your self what does smoking mean, does smoking have any advantage in my life you have to make assumption, you have to test or...