What's 1 times 2 / 1 + 2 times 3 / 1. + 99 times 100 / 1? Note that 1 times 2 is the denominator~

It's the same
=1-1/2+1/2-1/3+1/3-1/4+…… 1/98-1/99+1/99-1/100
Everything in the middle can be eliminated
=1-1/100
=99/100

Find the unknown X: (1) 1 / 3:1 / 6 = 8: X (2) 0.5x plus 1 and 1 / 8 times 6 = 7

1) 1 / 3:1 / 6 = 8: X
(1 / 3) x = 8 / 6
x=4
2) 0.5x plus 1 / 8 times 6 = 7
0.5x plus 27 / 4 = 7
0.5x = 1 / 4
X = 1 / 2

Fill in the largest number: 3 / 5 times 3 / 8 is less than 5 / 5 times 1 / 2 fill in the smallest number: 7 / 13 times 49 / 39 is less than 7 / 7 times 1 / 4

3 / 5 * 3 / 8 = 9 / 40 < 5 / X * 1 / 2, x < 100 / 9, the maximum value of X is 11;
7 / 13 * 39 / 49 = 3 / 7 < x / 7 * 1 / 4, x > 12, the minimum value of X is 12

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