Three mathematicky-type things for those who like them...

-1. All Derfs are Enajs. One-third of all Enajs are Derfs. Half of all Sivads are Enajs. One Sivad is a Derf. Eight Sivads are Enajs. The number of Enajs is 90. How many Enajs are neither Derfs nor Sivads?

-2. Two boats leave opposite sides of a still river (no cross-currents) simultaneously and head straight for the opposite bank. meeting at 300 yards north of the south bank. When they each reach the opposite bank they do an about-turn (in their own time) and the second time they meet they are 400 yards south of the north bank. Assuming both boats maintain a constant speed throughout, what is the width of the river?

-3. Mrs Worrier put three little bags on the table. "Hey, pudgy" she said (she was a great loss to the Diplomatic Corps, I've always thought). "Look at the different types of chocolate I bought with the two pounds I found in the pocket of your jeans when I washed them."

"What did they cost?" said I, ever the tight-fist.

"Well", she said,. "I got five ounces of one kind, six of another, and seven of the third. One cost nine pence an ounce less than what I'd have paid for an ounce of each of the other two. There was no change from the money that I found in your jeans. Oh - and one type of chocolate was 10 pence per ounce." (That's her all over - incapable of giving a straight answer to a straight question)

What was the cost of the other two types of chocolate?