How many quarts of pure antifreeze must be added to 6 quarts of a 30 % antifreeze solution to obtain a 40 % antifreeze solution?

X quarts of pure antifreeze

6*0.3 = 1.8 quarts antifreeze in a 30 % antifreeze solution

(x+6) - We are going to get  a 40 % antifreeze solution

0.4*(x+6) - quarts antifreeze in a 40 % antifreeze solution

x quarts of pure antifreeze +  1.8 quarts antifreeze in a 30 % antifreeze solution= = 0.4*(x+6)  quarts antifreeze in a 40 % antifreeze solution,
so we can write an equation

x+ 1.8 =  0.4*(x+6) 
x+1.8 = 0.4x +2.4
0.6x = 0.6
x=1 quart of pure antifreeze