The task is to generate the correct answer for a given math problem.

[Q]: Problem: from a pack of cards , two cards are drawn one after the other , with replacement . what is the probability that the first card is a black card and the second card is a king ?
[A]: 1 / 26


[Q]: Problem: if 4 xz + yw = 10 and xw + yz = 20 , what is the value of the expression ( 2 x + y ) ( 2 z + w ) ?
[A]: 50 .


[Q]: Problem: a train running at the speed of 60 km / hr crosses a pole in 15 seconds . find the length of the train .
[A]:
250