Copying Formula's

I have this formula:

=(C4/100)*D31

When I drag out and try to apply the same formula to the rest of the column I get:

=(C5/100)*D32
=(C6/100)*D33
=(C7/100)*D34 etc etc....

The question is how can I copy the formula so it always mulitplies by D31?

I have had to manually go through the formulas changing the D columns all to 31, VERY TIME CONSUMING I am sure you can imagine!

There must be an easier way!

Thanks