In a single writing, a painter contracted with a farmer to paint three identical barns on her rural estate for $2,000 each. the contract provided for the farmer's payment of $6,000 upon the painter's completion of the work on all three barns. the 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 the farmer obligated to make the $4,000 payment?