Given, c varies directly as d-3. So, the equation will be:
c= k(d-3) where k is constant of variation.
If c=16 when d=7. So, plug in these values in the above equation. Hence,
16=k (7-3)
16=k*4
16/4= k
k= 4
Now plug in k=4 in the above equation to get the equation of this variation.
c=4(d-3).
We need to find c when d=10. So, plug in d=10 in the above equation to get the value of c. So,
c =4(10-3)
=4*7
=28
So, c=28.