Two ways (I think). A nested if statement
if {queuetime} = 0 and {gettoagent} >= 40 then {gettoagent}-40
else
if {queuetime} > 0 then {queuetime}
else 0
else 0
Or an switch statement
switch (queuetime} = 0 and {gettoagent} >= 40 , {gettoagent}-40, {queuetime} > 0, {queuetime}, true, 0)
I hope this helps.