Maths

Let y km/h the original speed of Peter.

Time taken = 20/y h
Time taken when his speed decreases by 1 km/h = 20/(y-1) h

20/(y-1) - 20/y = 1
Multiply both sides by y(y - 1):
20y - 20(y - 1) = y(y - 1)
20y - 20y + 20 = y2 - y
y2 - y - 20 = 0
(y - 5)(y + 4) = 0
y = 5 ooro y = -4

Ans: The original speed of Peter is 5 km/h.
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