A Unit 2 work question

[VCE_2012]

Could someone please make sure that the book is wrong or I'm going crazy
For those that have the Nelson 4th ed, its on pg 252 Q5
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a) Calculate the work done when pulling a sledge a distance of 15.0 m along a smooth surface, while exerting a force of 120 N on a rope that makes an angle of 30 degrees with the horizontal. (<---- this one is fine)

b) What work would have been done of the surface had been rough and a constant frictional force of 45 N retarded the motion?

The book's answers are 1.6x10^3 J for both. I got to two decimal places (1403.46 Nm or J) for 'b)'.