Equation:
Total  =  Band Only  +  Orchestra Only   +   Both
Band Only   +   Orchestra Only    =    80%
Band  Only   =    50%
Orchestra Only    =     30%
Therefore:
Band   =   100%    -    Orchestra Only  30%   =   70%
70% of total students  =    119
Total students     =     170
Number of Students in Orchestra Only:
= ===>   30%  of   170   =   51
Solution for what is 30% of 170: Â
170 / x   =   100 / 30
(170 / x)  * x  =  (100 / 30)  * x    (we multiply both sides of the equation by x)
170   =   3.33333333333 *  x     (we divide both sides of the equation by )(3.33333333333)   to get   x
170 / 3.33333333333 Â Â Â Â Â = Â Â Â Â Â x Â
51 Â Â Â Â = Â Â Â Â Â x Â
x    =      51
now we have:
30% of   170   =    51
  Â
Therefore your,  Answer is  ========> 51 students is in the Orchestra only.
Hope that helps!!!!! Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â : )