a, (85 + 15) x y = 4000 c,
100 x y = 4000
y = 4000 : 10
y = 40
b; 18 + y + y + y = 48 x 25
18 + (y + y + y) = 1200
18 + 3y = 1200
3y = 1200 - 18
3y = 1182
y = 1182 : 3
y = 394
c; 64 x y - y x 13 - y = 43
y x (64 - 13 - 1) = 43
y x (51 - 1) = 43
y x 50 = 43
y = \(\dfrac{43}{50}\)