First off, let's convert the percentages to decimal format, so our 77% turns to 77/100 or 0.77, and our 55% turns to 55/100 or 0.55 and so on
now, the sum of both salines, must add up to the 77% mixture, let's say is "y"
so, 11 + 4 = y, and whatever the concentration level is, must also sum up to the mixture's concentration of 77%
anyway thus

solve for "x"
Answer:
9
Step-by-step explanation:
you can always use guess-and-check:
124 seems rather high
(6)P(2) = 6!/4! = 6x5 ≠ 72
(9)P(2) = 9!/7! = 9x8 = 72
Last one is your answer
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