A golf calculator? 29.

C:\test>perl -le"print eval while<>;"
2+2
4
3*5
15
$a=3**2+4
13
$a/2
6.5
(1+(2*3)+4)/($a%3)
11
^Z

C:\test>
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Or with a Log₁₀ function: 53. (Note: capital L)

C:\test>perl -le"sub Log{log(pop)/log(10)}print eval while<>;"
Log(100)
2
10**2
100
Log(30)+log(40)
5.1660007088336
10**(Log(30)+Log(40))
1200
exp(log(30)+log(40))
1200
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