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Click cell B4, then cell C4.
Notice the formulas in the Formula Bar.
Both formulas use cell E8 in the Numbers Per-Frog sheet.
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Click cell C4, then place the cursor on the Fill Handle.
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Drag it to cell M4.
The worksheet should look like this:

By employing an absolute reference—using the $ symbol to make each formula use the same cell containing the frog price—the formula stays accurate for all months.
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Edit the formula in cell B5 in the Revenue sheet to make cell G6 in the Numbers Per-Frog sheet an absolute reference.