There are 2 drivers, one in each bus. They are tipping each bus driver $20. Since there are 2 drivers, together they are tipping the bus drivers $40. Subtract $40 from the money they raised up.
$240 - $40 = $200 -> Money spent altogether for the bus rental
Since they are renting 2 busses, divide the bus money ($200) by 2.
$200 รท 2 = $100
ANSWER: They can spend $100 on each bus rental.