Algebra

Add the equations

2a = 10

a = 5

5 ( b + 1) = 250

b + 1 =  250 / 5

b + 1  =  50

b = 49

To cancel $ab$ you can add the equations together:

 $2a=10 \Rightarrow a=\color{blue}{\boxed{5}}$ 

Now plug in $a$ into one of the equations to get:

$5+5b=250 \Rightarrow 5b=245 \ \text{so} \ b=\color{blue}{\boxed{49}}$.

I hope that helped!