Germany was far ahead of the Allies in using heavy indirect fire. The German Army employed 150 mm  and 210 mm  howitzers in 1914, when typical French and British guns were only 75 mm  and 105 mm . The British had a 6-inch  howitzer, but it was so heavy it had to be hauled to the field in pieces and assembled. The Germans also fielded Austrian 305 mm  and 420 mm  guns and, even at the beginning of the war, had inventories of various calibres of Minenwerfer, which were ideally suited for trench warfare.
Question: How many mm smaller were the smallest French & British guns compared to the smallest German howitzers?
Answer: 30