I want to solve this integer-linear programming problem. There are four variables (xp,xq,xifp,xifq)
Two variables are nonnegative and integer (xp,xq)

the other two variables are binary(xifp,xifq)
 

When executing this command, maple says that the problem appears to be unbounded

LPSolve(750*xp+250*xq-50000*xifq,maximize=true,
{20*xp+30*xq<=2600,
30*xp+20*xq<=1000,
20*xp+30*xq<=2000,
xq<=99999*xifq,
xq>=20*xifq,
xp<=99999*xifp,
xp>=30*xifp,
xp>=0,
xq>=0},
integervariables={xp,xq},
binaryvariables={xifp,xifq}
);

and gives this solution[0, [xifp = 0, xifq = 0, xp = 0, xq = 0]]

however if i set a range for the variables xp,xq from 0 to 99999 for example

LPSolve(750*xp+250*xq-50000*xifq,maximize=true,
{20*xp+30*xq<=2600,
30*xp+20*xq<=1000,
20*xp+30*xq<=2000,
xq<=99999*xifq,
xq>=20*xifq,
xp<=99999*xifp,
xp>=30*xifp,
xp>=0,
xq>=0},
integervariables={xp,xq},
binaryvariables={xifp,xifq},
xp=0..99999,
xq=0..99999
);
maple returns this solution 

[49950000, [xifp = 1, xifq = 1, xp = 50000, xq = 50000]]

but if xp=50000 and xq=50000 this doesnt satisfy the first constrain
because 20*xp+30*xq</=2600
why does maple gives this mistaken solution, how can i solve it ?
please help,thanks for reading