Question 17
In the base ten number system the number $526$ means $5\cdot10^2+2\cdot10+6.$ In the Land of Mathesis, however, numbers are written in the base $r.$ Jones purchases an automobile there for $440$ monetary units (abbreviated m.u.). He gives the salesman a $1000$ m.u. bill, and receives, in change, $340$ m.u. The base $r$ is: