1) 6:5=x:75; 2) a:1/50=5/57:1/2; 3) x/0,8=15/4; 4) 5-y/4=3/7.

Решение:

1. \( 6 : 5 = x : 75 \)
   \( x = \frac{6 \cdot 75}{5} = \frac{450}{5} = 90 \)
2. \( a : \frac{1}{50} = \frac{5}{57} : \frac{1}{2} \)
   \( a \cdot 50 = \frac{5}{57} \cdot 2 = \frac{10}{57} \)
   \( a = \frac{10}{57 \cdot 50} = \frac{1}{57 \cdot 5} = \frac{1}{285} \)
3. \( \frac{x}{0.8} = \frac{15}{4} \)
   \( x = \frac{15 \cdot 0.8}{4} = \frac{12}{4} = 3 \)
4. \( \frac{5-y}{4} = \frac{3}{7} \)
   \( 7(5-y) = 4 \cdot 3 \)
   \( 35 - 7y = 12 \)
   \( -7y = 12 - 35 \)
   \( -7y = -23 \)
   \( y = \frac{-23}{-7} = \frac{23}{7} \)

Ответ: 1) x = 90; 2) a = 1/285; 3) x = 3; 4) y = 23/7.