Question 4
A train travels from $A$ to $B$ at a constant speed. After $70$ km, the driver discovers an engine fault and proposes reducing the speed by $25\%$. The conductor calculates that this would make the train arrive $1$ hour $20$ minutes late, beyond the permitted limit. He therefore asks the driver to continue at the original speed for another $40$ km before making the proposed reduction. The train then arrives $1$ hour late. Find its originally intended speed.