3) a)x/30 + x/24 + 1 = 5 1/2
<=> x(1/30+1/24) = 9/2
<=> x*3/40 = 9/2
<=> x = 60
b) 60-x = 2(80-3x)
<=> 60-x = 160 - 6x
<=> 5x = 100
<=> x = 20
c) 3/4(x-5) + 15/50-2x^2 = -7/6(x+5)
<=> 9(x+5)/12(x^2-25) + 90/12(25-x^2) = -14(x-5)/12(x^2-25)
<=>9(x+5)-90+14(x-5) = 0
<=> x = 5 ( loại ĐKXĐ )
d) 35x+45(x-2/5) = 90
<=> 35x+45x-18 = 90
<=> 80x = 108
<=> x = 27/20