Question

1/2(4a+10b) = c      solve for b 

Answer 1

1/2 (4a + 10b) = c

Multiply each side by  2 :        4a + 10b = 2c

Subtract 4a from each side:      10b = 2c - 4a

Divide each side by  10 :           b  =  1/5 c  -  2/5 a 

Answer 2

$\frac{1}{2}(4a+10b)=c \\\\ \frac{1}{2}*2(2a+5b)=c \\\\ 2a+5b=c \\\\ 5b=c-2a \\\\ \boxed{b=\frac{c-2a}{5}}$