Svetlana Z. answered 11/02/15
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Female Engineer, UC Berkeley PhD, 30+years industry 1700+ hours tutor
M is the total that Marty's plumbing charges
P is the total that physicians plumbing charges
H1 is the hourly rate of Marty's
H2 is the hourly rate of physicians.
F1 is the fixed rate that Marty's charges
F2 is the fixed rate that Physician's charges.
t is time
In general, the amount charged = (hourly rate) x t + fixed rate
So: M=H1 x t + F1
P = H2 x t + F2
We are given that physicians charges 10 less per hour than Marty's, so H2 = H1 - 10
physicians fixed service chage is 10 more than marty's: F2 = F1 + 10
Then P = (H1 - 10) x t + F1 + 10 = H1 x t - 10 x t + F1 + 10 = H1 x t + F1 + (10 - 10 x t)
since M = H1 x 10 + F1
P = M + (10 - 10xt)
and P - M = 10 - 10 x t
For a 5 hour job, the difference is 10 - 10 x 5 = -40
That means that Physicians would've charged $40 less