Answer:
39 numbers — the only number you can not get is 39
Explanation:
Take the digits 5, 4, 3, 2 and 1, in that order. Using those digits and the four arithmetic signs — plus, minus, times and divided by — you can get 1 with the sequence 5 - 4 + 3 - 2 - 1. You can get 2 with the sequence (5 - 4 + 3 - 2) x 1.
1=5-4+3-2-1;
2=(5-4+3-2)*1;
3=((5+4)/3) * (2-1);
4=(5+4-3-2)*1;
5=5*(4-3)*(2-1);
6=5-4+(3*2)-1;
7=5+4-3+2-1;
8=(5+4-3+2)*1;
9=5+4+3-2-1;
10=5+4+(3-2)*1;
11=5+4+3-2+1;
12=(5+4-3)*(2*1);
13=((5+4-3)*2)+1;
14=(5+4+3+2)*1;
15=5+4+3+2+1;
16=5+4+(3*2)+1;
17=5+(4*3)*(2-1);
18=5+(4*3)+2-1;
19=(5*4)-3+(2*1);
20=(5*4)-3+2+1;
21=(5*4)+3-(2*1);
22=(5*4)+3-2+1;
23=(5*4)+3*(2-1);
24=(5*4)+3+2-1;
25=(5*4)+3+(2*1);
26=(5*4)+3+2+1;
27=(5*4)+(3*2)+1;
28=5+(4*3*2)-1;
29=5+(4*3*2*1);
30=5+(4*3*2)+1;
31=((5*4*3)/2)+1;
32=5*(4+3)-2-1;
33=5*(4+3)-(2*1);
34=5*(4+3)-2+1;
35=5*(4+3)*(2-1);
36=5*(4+3)+2-1;
37=5*(4+3)+(2*1);
38=5*(4+3)+2+1;
(no equation for 39);
40=5*(4+3+2-1)