Решение:

1. а) $$\frac{13}{56} + y = \frac{34}{56}$$
   $$y = \frac{34}{56} - \frac{13}{56} = \frac{34-13}{56} = \frac{21}{56} = \frac{3}{8}$$
   
2. б) $$x - \frac{7}{90} = \frac{39}{90}$$
   $$x = \frac{39}{90} + \frac{7}{90} = \frac{39+7}{90} = \frac{46}{90} = \frac{23}{45}$$
   
3. в) $$\frac{27}{48} + \frac{15}{48} - a = \frac{17}{48}$$
   $$\frac{42}{48} - a = \frac{17}{48}$$
   $$a = \frac{42}{48} - \frac{17}{48} = \frac{42-17}{48} = \frac{25}{48}$$
   
4. г) $$b + \frac{14}{23} - \frac{3}{23} = \frac{20}{23}$$
   $$b + \frac{11}{23} = \frac{20}{23}$$
   $$b = \frac{20}{23} - \frac{11}{23} = \frac{20-11}{23} = \frac{9}{23}$$
   

Ответ:

1. $$\frac{3}{8}$$
2. $$\frac{23}{45}$$
3. $$\frac{25}{48}$$
4. $$\frac{9}{23}$$