Question 123566
 assuming 100% acid.... 


{100% }* x + [ 20% ]* (40 ml ) = ( 28 %) ( 40 + x ) 


where x = amount of acid in ml....


40 + x = total ml of 28% solution ...


convert % to decimal.... [ move decimal 2 times left on each ...]

1x + .20 * 40 = .28 ( 40 + x)


x + 8 = 11.2 + .28x


.72x = 3.2 ..... x = 4.44 ml