(y+x)*0.3=4.2①

y÷0.4= 12②

Then the solution of equation ② is y= 12*0.4 y=4.8.

Bring the value of y into equation ①, and the solution is 4.8*0.3+0.3x=4.2 x=9.2.

∴? =4.8

X=9.2