Умножим первое на 15, второе на 6:
{k + y - 5(k - y) = 15
{2k - y - 2(3k + 2y) = -25*6
{k - 5k + y + 5y = 15
{2k - y - 6k - 4y = -150
{6y - 4k = 15
{-4k - 5y = -150
{-4k = 15 - 6y
{15 - 6y - 5y = -150
{k = (6y - 15)/4
{-11y = -165
{y = 15
{k = (90 - 15)/4 = 18,75