Carol downloads x songs at 99 cents each , and y e-books at $2.99 each. Altogether she buys 11 items , where an item is a song or an e-book . The total amount of money she spends on this transaction is $22.89 . Solving which of the following equations correctly yields y , the number of e-books ?

Solve each of the following differential equations cos y "" dx +(1+2e^(-x)) sin y "" dy=0 .

The principal sanctioned a certain amount of money to the librarian to purchase some mathematics books costing Rs90 each . She could buy 80 books . Then she approached to a publisher who offered her a 20% discount . Find the number of copies of mathematics books which she could buy from the publisher for the sanctioned money.