Respuesta :
4+5=9 because your adding hold up 5 fingers in put 4 more that's 9
⇒4+5
=4+4+1----[5=4+1]
=2 × 4 +1
=8+1
= 9
→4+5
=(5-1)+5
=5+5-1
=10-1
=9
Addition Doubles Fact used
→4+4 =8
or
→5+5=10
