RUDI’S face crinkles up as he works on an application question:

"Ari’s book costs Rp500 more than Lala’s book. Ari buys two books and Lala buys three books. What is the cost of one of Ari’s books and one of Lala’s books if all the books together cost Rp9,000?"

Rudi fiddles with all the possibilities. Although he finally managed to answer the question, he needed more than 10 minutes to do so. A teacher at