Chart read-off Numerical Reasoning Question

Read values against the gridlines carefully; for stacked segments, subtract the boundary values to get the segment itself. The correct answer is 20. Traps to avoid: 10 comes from the "one gridline too low" error; 50 comes from the "read adjacent category" error; 25 comes from the "misread between gridlines" error; 30 comes from the "one gridline too high" error.