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Posted by on Thursday, August 4, 2011 at 10:28am.

Show that one and only one out of n,n+4,n+8,n+12,n+16 is divisible by 5,where n is any positive integer. A two digit number is obtained by either multiplying the sum of the digits by 8 and then subtracting 5 or by multiplying the differences of the digits by 16 and then adding 3.help me immediately.

  • Math help please - , Thursday, August 4, 2011 at 11:13am

    Hints for first part:
    0 mod 5 = 0
    4 mod 5 = 4
    8 mod 5 = 3
    12 mod 5 = 2
    16 mod 5 = 1

    Use (a+b)mod 5 = (a mod 5 + b mod 5)mod 5.

  • Math help please - , Thursday, August 4, 2011 at 11:34am

    write the sequence as
    (n) + 0 , (n)+4, (n+5) + 3 , (n+10) + 2, (n+15) + 1

    dividing any number n by 5 will leave remainder of 1, 2, 3, 4, or 0 (a remainder of zero will give an exact divsion)

    The + ? at the end would be the possible remainders for different values of n, and all possible remainders show up.
    e.g
    if n = 1, the 2nd, or (n) + 4 is divisible by 5
    if n = 2 , the 3rd or (n+5)+3 is divisible by 5
    if n = 3, the 4th, or (n+10) + 2 is divisible by 5
    if n = 4 , the 5th, or (n+15) + 1 is divisible by 5
    if n = 5 , 1st, or n is divisible by 5
    if n = 6 , the 2nd, or (n) + 4 is divisible by 5
    and the cycle keeps going like that.
    Notice that for any value of n, there is only one of the expressions that would be divisible by 5


    Your second question:
    let the unit digit be x, let the tens digit by y
    the number would be 10y+x
    8(x+y) - 5 = 16(x-y) + 3
    8x + 8y - 5 = 16x - 16y + 3
    8x - 24y = -8
    x = 3y - 1

    let y = 1 , x = 2 , number is 12
    let y = 2, x = 5 , number is 25
    let y = 3, x = 8 , number is 38

    test:
    for 12, is 8(1+2)- 5 = 16(2-1) + 3 ? Yes
    for 25, is 8(2+5) - 5 = 16(5-2) +3 ? yes
    for 38, is 8(3+8) - 5 = 16(8-3) + 3 ? Yes

    the two digit number could be 12, 25, or 38

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