Question 301206
let the quantity of 30 % acid required be x liters

the 18% acide required will be 500 - x liters

we need 500 liters 25% acid solution 

0.3x+0.18(500-x) = 0.25 * 500

0.3x+90-0.18x=125

0.12x=35

x= 35/0.12

291.66 liters of 30% acid is required.

balance will be 18% acid