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Let the original cost per person  = C

So......the revenue collected is just   20C

When 10 more join, the cost per person is  (C - 1.50)

And the revenue is 30 (C - 1.50)

And the revenue (cost for renting the bus)  is the same in both cases....so

20C  =  (30) (C - 1.50)    simplify

20C  = 30C  -  45   subtract  30C from both sides

-10C  = - 45

C = 45/10  = 4.50

So.....the cost of renting the bus  =   20 C  =  20 * 4.50  =   $90