Spice A is 30% salt. We do not know how much of this spice to add, so we'll call it x. The x is a placeholder for a number So on your paper you might write x = amount of spice that is 30% salt (amount in ounces) 30% of x = 0.30*x is the amount of pure salt we have in spice A
Spice B is 65% salt. We have 175 ounces of this stuff. 65% of 175 = 0.65*175 = 113.75 ounces are pure salt So we have 0.30x+113.75 ounces of total pure salt from the two spices combined
The total mix is x+175 ounces. We want 50% of the total mix to be pure salt, so 0.50*(x+175) is the expression that represents this.
Set the expression 0.30x+113.75 and 0.50*(x+175) equal to each other. Solve for x 0.30x+113.75 = 0.50*(x+175) 0.30x+113.75 = 0.50*x+0.50*175 0.30x+113.75 = 0.50*x+87.5 0.50*x+87.5 = 0.30x+113.75 0.50*x-0.30x = 113.75-87.5 0.20x = 26.25 x = 26.25/0.20 x = 131.25
