Final answer:
To multiply the number of reeds bought by the price of a single reed and copy the formula, use an absolute reference to the cell containing the quantity of reeds bought.
Step-by-step explanation:
To multiply the number of reeds bought (c4) by the price of a single reed (g4), you can use the formula =C4*G4 in cell D4. To change this formula so it can be copied to cells D5:D18 while keeping the reference to C4 constant, you can use the absolute reference $C$4. So the formula for cell D5 would be =$C$4*G5 and for D6 it would be =$C$4*G6, and so on.
\