My father is 47 years old and my son is 21 years old. How many years ago, my father was three times as old as my son?

Suppose that the father was three times the age of his son x years ago
47-X=3×（21-X）
47-X=63-3X
2X=63-47
2X=16
X=8
So eight years ago, father 39, son 13

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1. 1 × 2 / 1 + 2 × 3 / 1 + 3 × 4 / 1 + 4 × 5 / 1 +... + 39 × 40 / 1
2. 40 2／3•39 1／3=?
3. 2/1+(3/1+3/2)+(4/1+4/2+4/3)+.+(1/40+2/40+...+38/40+39/40) One or two steps in the process, and the answer
4. 1/2＋（1/3＋2/3）＋（1/4＋2/4＋3/4)＋.＋（1/40＋2/40＋.＋38/40＋39/40） 1/2＋（1/3＋2/3）＋（1/4＋2/4＋3/4）.＋（1/60＋2/60＋.＋58/60＋59/60）
5. 2\1+(3\1+3\2)+(4\1+4\2+4\3)+.+(40\1+40\2+.+40\38+40\39)
6. 1 / 2 + (1 / 3 + 2 / 3) + (1 / 4 + 2 / 4 + 3 / 4) +. + (1 / 40 + 2 / 40 +... + 38 / 40 + 39 / 40) It can be seen that the number equidistant from the first place in each bracket is added, so the items in brackets can be arranged in reverse order and added Who can help me
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