GMAT Math

A 400-meter runner spent 4 more seconds on the first half of the distance than he did on the second. If the runner finished the 400-meter distance in 50 seconds, what was his average speed for the first 200 meters?(A) 24 1/3 (B) 25 1/3 (C) 26 2/3 (D) 27 2/3 (E) 29 1/2 all in kmph

If there are 10 liters of a 20%-solution of alcohol, how much water should be added to reduce the concentration of alcohol in the solution by 75% ? (A) 25 liters (B) 27 liters (C) 30 liters (D) 32 liters (E) 35 liters

what units are the answers in? i'd assume same as the question, meters/sec. who runs 24m/sec?

THe answer in the first question don't make sense. 400 meters in 50 seconds is 8 meters per second. THe answer is going to be close to 8, not 25.

Total time 50 seconds, the first section is four seconds longer than the second section so we have
x + (x+4)= 50. That means our runner spent 27 seconds on the first 200 meters and 23 seconds on the second 200 meters. The rate on the first 200 is 200/27 meters per second. (approx. 7.4 mps)

For the second problem we have 10 liters total and 20% alcohol = 2 liters.

We are not changing the amount of alcohol so we will still have 2 liters in the new mixture.

Reducing the concentration by 75% means reducing 20% by 75%, leaving us with a target of 5% alcohol.

FOr 2 liters to be 5% of the total, the total must be 40 liters.

We started out with 10 liters and ended up with 40 so we added 30. Answer C

Its Km per hour i guess we have to convert units

correct: 7.4 meters per second *60 = 444 meters per minute * 60 = 26,640 meters per hour = approximately 26 2/3 kmph