Step-by-step explanation:
so, we are mixing A and B for the drug.
a = amount of A
b = amount of B
a + b = 553 ml
a/b = 4/3
a = 4b/3
we use the last equation in the first and get
4b/3 + b = 553
4b/3 + 3b/3 = 553
7b/3 = 553
7b = 553×3 = 1659
b = 1659 / 7 = 237 ml
a + b = 553
a = 553 - b = 553 - 237 = 316 ml
the chemist needs 316 ml of A.