A key to gough's practical arithmetic : In which all the examples necessary for a learner are wrought at large; and also solutions given of all the questions for exercise in the various rules. Designed chiefly for the ease of schoolmasters, and assistance of such as are deprived of the advantage of a teacher; which, together with the practical treatise, so generally approved of, must furnish the most useful and extensive system of arithmetic with which the public have hittherto been favoured. By Robert Telfair, of the Belfast Academy.