Question 922704
let x be the no of correct answers you did
let y be the no of wrong answers you did
 so x+y = 150 ----eq(1)
 marks you got is 90
          so   x*(1)+y(-1)= 90
                x-y =90
                move y to the right
                   x = 90+y
                  put this x= 90+yin x+y= 150    
                                      (90+y)+y= 150
                                        90+2y = 150
                                subtract 90 on both sides
             90+2y -90 =150-90
                  2y = 60
             divide with 2 on both sides
                    2y/2  = 60/2
                        y= 30 
                          x= 90+y= 90+30 =120 
so no of correct answers you made  = 120