In a single writing, Painter contracted with Farmer to paint three identical barns on her rural estate for $2,000 each. The contract provided for Farmer's payment of $6,000 upon Painter's completion of the work on all three barns. Painter did not ask for any payment when the first barn was completely painted, but she demanded $4,000 after painting the second barn. Is Farmer obligated to make the $4,000 payment? 
No, because Farmer has no duty under the contract to pay anything to Painter until all three barns have been painted.  No, because Painter waived her right, if any, to payment on a per-barn basis by failing to demand $2,000 upon completion of the first barn.  Yes, because the contract is divisible.  Yes, because Painter has substantially performed the entire contract. 