1. The problem gives the following information: "a" varies jointly with "b" and "c" and inversely with "d".Â
 2. You have that the values of "a", "b", "c" and "d" are:
 a = 400
 b = 16
 c = 5
 d = 2
 3. Keeping this on mind, you must calculate the constant of proportionality "k", as below:
 a=kbc/d
 3. When you clear "k", you have:
 k=ad/bc
 k=(400)(2)/(16)(5)
 k=10
 4. Therefore, you have:
 a=kbc/d
 a=10bc/d
 The answer is:
 a=10bc/d
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