Mr. and Mrs. Birch are both over 65 years of age and are filing a joint return. Their income for the current year consisted of the following:
Taxable interest: $6,000
Taxable dividends: $8,000
Social Security payments (Mr. and Mrs. Birch combined): $15,000
Tax-exempt interest: $4,000
Taxable pension: $15,000
They did not have any adjustments to income. What amount of Mr. and Mrs. Birch's Social Security benefits is taxable?