Solution key :  level 7. 

 - Boston:   Houghton Mifflin Company,   1987.  
 - 219 p. ;  24 cm. 
 - Houghton Mifflin Mathematics .
<br/><br/><label>ISBN: </label>0395387388 
<br/><br/><label>Subjects--Topical Terms: </label><br/>Mathematics<br/>Mathematics--Problems, exercises, etc.
<br/><br/><label>Dewey Class. No.: </label>510