There will be four steps 1st two of 3 miles each next will be of 2 miles then final step will be of 7 miles.
STEP1: Camel will first eat 1 banana for next 1 mile then it will take 15 of them (29 are still left). Camel will finish 2 more for forward 3 miles then she will leave 10 of them and she will keep 3 for return journey. it will repeat the step one more time. Now she is with 13 bananas she will simply go to the first 3 miles eating three of them.
STEP2: Now she is with 30 bananas .she will repeat same steps for next three miles. After these 6 miles she is left with 21 bananas.
STEP3: Now it will do the same thing for next 2 miles .
STEP4: Now she is at 8th mile with 15 bananas she will take all of them for next 7 miles. finally she is with 8 bananas at the city.