Tính nhẩm

$\eqalign{ & a)\,\,80\,\, - \,\,20\,\, - \,\,10\,\, = \,...\, \cr  & \,\,\,\,\,80\,\, - \,\,30\,\,\, = \,...\, \cr  & b)\,70\,\,\, - \,\,\,30\,\, - \,\,20\,\, = \,...\, \cr  & \,\,\,\,\,70\,\, - \,\,50\,\,\, = \,...\, \cr  & c)\,\,90\,\, - \,\,20\,\, - \,\,20\,\, = \,... \cr  & \,\,\,\,\,\,90\,\, - \,\,40\,\,\, = \,...\, \cr}$

Khoanh vào chữ đặt trước kết quả đúng:

44 – 4 = ?