The excercise goes like" there are 20 books on the lowest shelf, 18 on next one, 16 on next one etc., till the top shelf on which there is 4 books. how many books are on this bookshelf? "

K so bottom has 20 and each time it goes up you the shelf has 2 less books, so to find how many shelves there are. There are 9 shelves so we add the number of books of these 9 shelves 20+18+16+14+12+10+8+6+4=108

THere are 108 books

Hope that helpes srry if took so long i had to get up for a sec :)

1. isolate the variable, in this case, d. subtract 35.5 on both sides of the equation.2. 52 subtracted by 35.5 is 16.53. therefore your answer would be d= 16.5this is the basics of pre algebra, isolating variable through adding, subtracting, multiplying, and dividing, if you have any other questions to ask just comment!