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What kind of bond pays interest which is exempt from tax?

For the rule of 78, for a 12-month period, the last term in the sequence is 12 and the series sums to 78. for an 10 month period, the last term is and the series sum is . for a 15 month period, the last term is and the series sum is . for a 20 month period, the last term is and the series sum is

Haley buys 9 pounds of apples for $3. how many pounds of apples can she buy for $1?