A tank-flow problem uses the same amount–rate–time reasoning as work, but water can leave as well as enter. You will track the water already present, use signs for changes in stored volume, and stop at the requested first-full or first-empty event. You will combine inlets, subtract an outlet or leak, convert volume/time units, infer a constant leak from two fill experiments, and handle a valve change. You will also recognise zero or negative net accumulation and decide whether missing information matters before the target event. These are idealised arithmetic models. Unless a question deliberately withholds a condition, each stated rate remains constant over the stated volume range; flows add independently; tank capacity is fixed; there are no unmentioned flows or delays. Actual pressure-dependent flow needs a different model. Stored water must stay between empty and capacity.