请教各位两道数学题

小哎

DS:If a and y are positive integers,is xy a multiple of 8?
   1  the greatest common divisor of x and y is 10
   2  the least common multiple of x and y is 100

PS:  a prepared 4 different letters to be sent to 4 different addresses.For each letter , she prepared an envelope with its correct address. If the 4 letters are to be put into the 4 envelops at random, what is the probability that only 1 letter will be put into the envelop with its correct address?

小哎

还有一道：

If 500 is the multiple of 100 that is closest to x and 400 is the multiple of 100 that is closest to y , which multiple of 100 is closest to x+Y?
1   x<500
2   y<400