Let's represent the number of days they were on holiday with the variable "d".
We know that Lucas spent $260 per day and ended up with $240 left. Therefore, he spent a total of:
$260d + $240 = $260d - $(-$240) = $260d - $(-240)
We can also represent the amount spent by Mike as:
$180d + $720 = $180d + $720 - $0
We know from the problem that both of them started with equal amounts of money, so we can set these two expressions equal to each other:
$260d - $(-240) = $180d + $720 - $0
Simplifying this expression, we get:
$260d + $240 = $180d + $720
Subtracting $180d from both sides gives:
$80d + $240 = $720
Subtracting $240 from both sides gives:
$80d = $480
Dividing by 80 gives:
d = 6
Therefore, Lucas and Mike were on holiday for 6 days.