Maria drove at a steady speed for 5 hrs on the turnpike she

Maria drove at a steady speed for 5 hrs. on the turnpike. she then slowed her traveling speed by 15 miles per hour for traveling on secondary roads. the entire trip took 7 hours and covered 369 miles. what was her speed on the turnpike?

Solution

d=rt
369=5x+(7-5)*(x-15)
369=5x+2(x-15)
369=5x+2x-30
369+30=7x
399=7x
x=399/7
x=57 mph. for the first 5 hrs.
57-15=42 mph for the last 2 hrs.
Proof:
369=5*57+2*42
369=285+84
369