question

The first item of a new product took 2000 hours to manufacture (at a labour cost of 15 per hour). A 90% learning curve was expected to apply, and it was decided to establish a standard time as the time required to manufacture the 50th item of the product, rounded to the nearest hour. The 50th item actually took 980 hours.
What is the labour efficiency variance for the 50th unit and whether it is favorable or adverse.
I calculated the variance to be 631.7238 adverse, but at the back of the book it has been given as 645. So is my answer also correct or is it wrong.