不知你的X是在分母还是分子
如果是分子：
(2/5)X + 21 = (3/4)X
(2/5)X - (3/4)X = -21
0.4X - 0.75X = -21
-0.35X = -21
X = -21 ÷ (-0.35)
X = 60
如果是分母：
2/5X + 21 = 3/4X
2/5 + 21X = 3/4
21x = 3/4 - 2/5
21X = 15/20 - 8/20
X = 7/20 ÷ 21
X = 1/60