$a)787\stackrel{2}{-}\stackrel{1}{\overbrace{4\cdot 93}}=787-368=419$  

 

$b)2\stackrel{2}{\cdot }\stackrel{1}{\overbrace{\left(894+1566\right)}}=2\cdot 2460=4920$  

 

$c)431\stackrel{2}{+}\stackrel{1}{\overbrace{1240:2}}=431+620=1051$  

 

$d)\stackrel{1}{\overbrace{\left(3260-1648\right)}}\stackrel{2}{\text{:}}2=1612:2=806$