Cumulative Frequency A level Question

Could someone please help me understand the last part to the attached question. The markscheme says the answer is 'too low, should be 26 or 27'. I don't understand why.

See attached.

The intequartile range in part (i) is given by the difference between the two vertical green lines.

With the new information, the cumulative frequency graph would be a straight line between 35 and 55 marks. So, lower quartile will be shifted to the left (vertical blue line), and the revised interquartile range will be the difference between the green line on the right and the vertical blue line. I.e. it's greater. So, the original estimate was too low.