@PhiNotPi‘s solution is quicker, but this is probably what schools teach. If you took Pre-Algebra, you might have run into a problem-solving method called substitution. Important parts in bold.
Describe the problem: 750 = A + B + C (it comes out and tells you this)
You got two ratios, so cross-multiple them (this looks nicer written out). B is in both ratios, so let’s solve each ratio for B. All you need to do is cross-multiply:
First ratio:
A/B=5/2
2A=5B
A = 2.5B
basically, we’re saying A is 2.5 times bigger than B
Second Ratio:
B/C = 3/2
2B=3C
C = ⅔B
that’s a 2 divided by 3, in case your font is tiny – we’re saying C is two-thirds the size of B
Using this information in bold, we can substitute A and C in a way that is described in terms of B and use basic algebra to find the value of B:
750 = A + B +C
750 = 2.5B + B + ⅔B
750=(25/6)B
B = 180
this is how much salt is in container B
Then you can pretty easily solve those two cross multiplied problems in bold from before:
A = 2.5B = 2.5(180) = 450
C = ⅔B = ⅔(180) = 120
And we already figured out B = 180