For the distribution 25-35: 6, 35-45: 10, 45-55: 12, 55-65: 8, 65-75: 4 with $a=50$ and $h=10$, the value of $\sum f_i{u}_i$ is:

Class marks 30, 40, 50, 60, 70 give $u_i=-2,-1,0,1,2$, so $\sum f_i{u}_i=-12-10+0+8+8=-6$. (The mean then follows: $50+10\left(\frac{-6}{40}\right)=48.5$.) "6" reverses the sign of $u_i$; "40" is $\sum f_i$, not $\sum f_i{u}_i$; "-60" multiplies by $h=10$ too early — $h$ belongs in the mean formula, not in $\sum f_i{u}_i$.