Упростите выражение: a) 3x+15y-2x-20y; б) -4(x-3)+2(x-5)-3(2x+4); в) -2(0,5x-1,4y)+2x+3,2y.

a) 3x + 15y - 2x - 20y = (3x - 2x) + (15y - 20y) = x - 5y.
б) -4(x - 3) + 2(x - 5) - 3(2x + 4) = -4x + 12 + 2x - 10 - 6x - 12 = (-4x + 2x - 6x) + (12 - 10 - 12) = -8x - 10.
в) -2(0,5x - 1,4y) + 2x + 3,2y = -x + 2,8y + 2x + 3,2y = (-x + 2x) + (2,8y + 3,2y) = x + 6y.