Robert can swim the first 50 meters of a race in 1 minute. Then he slows down by 12 seconds for each of the next 50 meters of a race. How long will it take Robert to swim a 400 meter race?

Since you already know that the first 50 meters took 1min you can subtract

400meters-50meters=350meters

350meters:50meters=7minutes

It increases at 12 sec./min for 7mins

1.12,1.24,1.36,1.48,2.00,2.12,2.24 = 11.56+1.00=12.56min