For this item, we represent first the proportionality constant by the variable k. Being directly proportional to both b and c, these variable will be in numerator and d will be in the denominator. That is,
a = kbc / d
Substituting the known values,
400 = k(16)(5) / (2)
The value of k is 10. The equation that models the relationship is therefore,
a = 10bc/d