Hi Mary, here's what PJ has to say about converting from a 10" X 15" pan:  If you’re baking bars, substitute one 9” square pan (81”), plus one 8” square or 9” round pan (64”) for a total area of 145”. But if you’re making a cake, it’s probably a sponge cake that really does need to bake in that specific 10” x 15” pan, since it’s almost certainly going to be rolled up into a log. Other pan combinations (two rounds? A round and a square?) just won’t cut it.

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