Answer
Number of arrangements that can be made using four letters = 840
Work Step by Step
Total number of letters in the word COMBINE = 7
We need to arrange number of letters = 4
Number of arrangements that can be made using four letters = $n_{{P}{_{r}}}$ = $\frac{n!}{(n - r)! }$
$7_{{P}{_{4}}}$ = $\frac{7!}{(7 - 4)!}$ = $\frac{7!}{3!}$ = $\frac{7\times6\times5\times4\times3!}{3!}$ = $7\times6\times5\times4$ = 840