Explanation:
cup A = 60% of milk
 = 60% × 1600 ml
 = 960 ml
cup B = 1600 ml - 960 ml
 = 640 ml
milk drank from cup A
= 40% × 960
= 384 ml
milk drank from cup B
= 70% × 640
= 448 ml
how much more milk from cup B had been drank ?
= 448 ml - 384 ml
= 64 ml