What is the series you are referring too?
Divide the current with the previous
18/15 = 1.2
move the decimal point to the right two places to get the percentage
120% is your answer
hope this helps
40/10 = 4 x 3.5 = 16 litres
16/2 = 8
Therefore 8 litres of pure water and 8 kites of pure acid
