Question 241558
Usual speed is 12 km/hr
25% faster = 12 + .25*12 = 12*(1+.25) = 12*(1.25) = 15 km/hr
25% slower = 12 - x*12 = 12*(1-x) km/hr


first 60 km lap is done at 15 km/hr
second 60 km lap is done at 12*(1-x) km/hr


overall average is 12km /hr


15 * 60 + 12*(1-x) * 60 = 120 * 12


900 + (12-12x) * 60 = 1440


900 + 720 - 720*x = 1440


add 720*x to both sides of the equation and subtract 1440 from both sides to get:


900 + 720 - 1440 = 720*x


combine like terms to get:


1620 - 1440 = 720*x


combine further to get:


180 = 720*x


divide both sides of the equation by 720 to get:


x = .25


substitute in the original equation to get:


15 * 60 + 12*(1-x) * 60 = 120 * 12 becomes:


15*60 + 12*(1-.25)*60 = 120*12 which becomes:


900 + 12*.75*60 = 1440 which becomes


900 + 540 = 1440 which becomes:


1440 = 1440 which confirms the answer is good.


answer is:


x = .25 * 100% = 25%