TOW A Typo

One number among the problem sets...
I solved it, and compared my answer with the answer of the problem at the back of the school.
I didn't get it.
I solved on it for 30 minutes...
My patience ran short and asked it to Kirk, my good friend who participated in the International Mathematics Olympiad.
He had the same answer with me.

He was sure of his answer and explained thoroughly, which was how I thought it should be...(note it's not that I am good, but the problem was very similar with other problems I solved in my life)

So, we decided on one thing. Typo...

Our answer was 6 Taken 3, but the answer at the back of the book said 6 Taken 2.

The authors are singaporeans. If there are more typos in this book which we will use for the rest of the semester, boy, they will be sorry...