Can a proportion have a zero denominator?

No. Division by zero is undefined, so every denominator must be nonzero.

How do I check my answer?

Substitute the result into both ratios and compare the cross-products.

A clear method for finding an unknown in two equal ratios, checking the result, and avoiding zero denominators.

To solve a/b = c/d, cross-multiply to get ad = bc, then divide by the coefficient of the unknown. The denominator values must be nonzero.

a/b = c/d → ad = bc

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