You can set up an equation 50=2x+3y where x is the number of carnations and y is the number of roses. Then, you can express x as x=25-3/2y. As x must be positive, you can write 25-3/2y>0, 25>3/2 y. So, y can be 2,4,6,8,10,12,14,16 and the corresponding x's giving u 8 combinations.