You need to find the total distance and the total time. The total distance is two times the distance to school, 2d.s=d/t=>t=d/sWhere s is speed, d is distance and t is time. So the time taken to get to school is:d/s1and the time taken to get back is:d/s2so the total time is:d/s1+d/s2=d(1/s1+1/s2)So, as speed = distance/time, divide the total distance by the total time:2d/d(1/s1+1/s2)=2/(1/s1+1/s2)=2/(1/20+1/40)=80/3≈26.7km/hr
Hello from MrBillDoesMath
Answer: + 4/7, 6 (13/14)
Discussion:
Let L be the larger of the two numbers and S the smaller of the two. Then the first sentence translates into this:
-6L - L = -4 =>
-7L = -4 => (divide both sides by -7)
L = - (-4)/(-7) = + 4/7 (*)
The second sentence translates to
2 (L + S) = 15 => (divide both sides by 2)
(L +S) = 15/2 => (subtract L from each side)
S = 15/2 - L = 15/2 - 4/7 (using (*) above)
So
S = 15*7/(2*7) - (4*2)/(7*2) (combine over common denominator of 14)
S = 105/14 - 8/14 = 97/14 = 6 13/14
Something is crazy here! While 2(L +S) = 15 -- check it out -- our larger number L ( =4/7) is actually smaller than the S number (6 13/14)! Maybe I should have called them Y and X, respectively, as you suggested!
Thank you,
Mr. B
