1. The problem gives the following information: "a" varies jointly with "b" and "c" and inversely with "d".
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