The real question is why should it be? Because that’d be easier? If you plug in x = 50, y = 42, and z = 35 (which are obviously in the ratio 50:42:35) the original equation clearly isn’t true.

50x equals 42y when x = 42 and y = 50, not x = 50 and y = 42. Both sides of the equation are equal to 50 * 42. If x = 42 and y = 50, what should z equal? (It won’t be an integer, but you can multiply everything by a constant so that it is afterwards).