Try this:

A1= 21+69+89+25+31
A2= 21*25
A3= 100/10
A4= 100/10*(10*10+10)
A5= 100/10*10*10+10

without the "=" sign.

1) Select cell  B1
2) Go to Insert>Name>Define
3) Type the name Result (can be any valid range name)
4) In the Refers to: box type: =EVALUATE($A1)
5) Click Add then OK.

Very important to select cell B1 and use a Relative Row reference for
$A1.
6) Simply enter =Result into cell B1 and copy  it down.