51. When a contract consists of reciprocal promises to be simultaneously performed, no promisor need perform his promise unless the promisee is ready and willing to perform his reciprocal promise.Illustrations (a) A and B contract that A shall deliver goods to B to be paid for by B on delivery. A need not deliver the goods, unless B is ready and willing to pay for the goods on delivery. B need not pay for the goods, unless A is ready and willing to deliver them on payment. (b) A and B contract that A shall deliver goods to B at a price to be paid by installments, the first installment to be paid on delivery. A need not deliver, unless B is ready and willing to pay the first installment on delivery. B need not pay the first installment, unless A is ready and willing to deliver the goods on payment of the first installment.