ATC_SR,10 and ATC_SR,30 show short-run average costs when the firm has a fixed capital level that minimizes the total costs of making 10 and 30 engines, respectively. With and , ATC_SR,10 and ATC_SR,30 overlap ATC_LR at the cost-minimizing points X and Z, respectively. However, X and Z are not at the lowest points on ATC_SR,10 and ATC_SR,30 because the output levels that minimize ATC_SR,10 and ATC_SR,30 can be produced more cheaply if capital inputs are flexible.