Find the sum of the numbers which are co-prime to 840 and are less than it.

A.40320

B.80640

C.120960

D.60480

B. 80640

$$
\begin
840= & 2^3 \times 3 \times 5 \times 7 \\
\phi(840) & =840 \times\left(1-\frac\right)\left(1-\frac\right)\left(1-\frac\right)\left(1-\frac\right) \\
& =840 \times \frac \times \frac \times \frac \times \frac \\
& =192
\end
$$
Sum of all numbers co-prime to n and less than $n=\frac \times \phi(n)$.

420*192= 80640