Bit late in seeing this, but:
X = W+R, where W is the no of White Roses and R is the no of Red.
The ratio of 80% W means W=4R, or 0.25W=R.
When you add the other roses:
Total new no of White/total new no of roses = 0.75
(76+W)/((76+W)+(48+R))=0.75 ;
substitute in either of the above values for R or W:
(76+W)/((76+W)+(48+0.25W))=0.75;
(76+W)=57+0.75W+36+0.1875W;
0.0625W=17
W=272
therefore R=272/4=68
X=272+68=340