The vet made 8 * 15 = $<<8*15=120>>120 in dog vet fees.
The vet made 3 * 13 = $<<3*13=39>>39 in cat vet fees.
The total vet fees were 120 + 39 = $<<120+39=159>>159.
The vet donated 1/3 of the fees, so they donated 159 / 3 = $<<159/3=53>>53 back to the shelter.
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