1+2-3-4+5+6-7-8+9+… +1997+1998-1999
The sum of each group is - 4
1999 / 4 = 499.3, a total of 499 groups
499x(-4)=-1996
Don't forget to add the last three and 1997 + 1998-1999 = 1996
(-1996)+1996=0
2 out of 9 times 9 out of 7 equals
Two out of seven
What's 25 times 9 and 7 / 8
Original formula = 25 * (10-1 / 8)
=250-25/8
=246 and 7 / 8