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23+22+21+… +1+0+1+2+3+… +54 = how much did you get in the end?
It's 1 + 2 + 3 +22+23+（1+2+3+… +53+54）=0.5*（1+23）*23+0.5*（1+54）*54=1761
Simple calculation of 3 / 4 * 31-1 / 4 * 21
1/4（31×3-21）=1/4×72=18
How to calculate 5 / 21 times 31 / 1 plus 3 / 4 times 31 / 21
21/5*1/31+4/3*21/31
Original formula = (21 / 5 * 1 / 21) (1 / 31 * 21) + 4 / 3 * 21 / 31
=1/5*21/31+4/3*21/31
=（1/5+4/3）*21/31
=23/5*7/31
161/155
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