It not HW, i need help on this kind of problems?

Question

A brand of grape drink that is 25% grape juice is mixed with another brand that is 75% grape juice. The result is 7 gallons of grape drink that is 35% grape juice. How much of the brand that is 25% grape juice was used?

can some1 plz provide an explanation for this

Answer 1

using trial and error

5 gallons of 25% and 2 gallons of 75%=40%

5.5 of 25% + 1.5 of 75% = 35.7 percent

5.6 of 25% and 1.4 of 75% = 35%

muliply 5.6  bottles x 25 =140

multiply 1.4 bottles  x 75 = 105

140+105=245

as an equation

a(25) + b(75) = 7(35)

subsitute for a and b (2 numbers taht equal 7)

245  / 7 =35