John averages 60 miles per hour (mph) the 6 hours he travels from his house to Ling's house. On his return trip, John experiences heavy traffic due to construction zones. He averages 36 mph the first 3 hours of his return trip. What is the average speed, in miles per hour, John must drive for the rest of the return trip for a return trip of 7 hours?

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The distance from John's house to Ling's house:

$$ d=rt $$ $$ d= 60(6) = 360 \text{ miles}$$

During the first 3 hours of the return trip, John travels:

$$ d=rt $$ $$ d= 36(3) = 108 \text{ miles} $$

For the last 4 hours, John must travel:

$$ d = 360-108 = 252 \text{ miles} $$

Traveling 252 miles in 4 hours:

$$ d=rt$$ $$ r= \frac{d}{t} $$ $$ r= \frac{252}{4} =\boxed{63}$$