Say, amount of order is x
7 truck 4 days can clear the order x
1 " 4 " " " " " x/7
1 " 1 " " " " " x/28.........................(i)
Similarly,
3 truck in 10 days can clear the order x
1 " " 1 " " " " " x/30..........................(ii)
(i) and (ii) equations are equivalent
So, x/28 $\alpha$ x/30
x/28 = c. x/30
or, c=15/14
4 days x work is done by 7 trucks
1 " x " " " " 7*4 "
5 " x " " " " 7*4 /5 "
and it is equivalent to previous task , i.e. 7*4 /5 c = 7*4 /5 * 15/14 = 6 days