A plumber charges $75 for a service call, plus a $40 per hour. If the plumber's bill was $235, then how many hours was he there?

4 Hours

40x - 75 = 235

         -75    -75

40x  = 160

40x/40 = 160/40

x = 4

4 hours

Step-by-step explanation:

235-75 one time fee, so subtract = 160

160/40 = 4

the 1st & last paras are fixed.

that leaves 3 to be put in any order

there are 3 choices for the 1st,

2 choices left for the 2nd

and just 1 choice for the last

multiply to get

3*2*1

= 6 ways