The number of non negative integral solutions of the equation \( x+y+z+t=10 \) when \( x \ge 2, z \ge 5 \) is
Show Hint
To handle constraints of the form \( x_i \ge k \), simply subtract the sum of all lower bounds \( k \) from the total sum \( N \) on the RHS. The new sum becomes \( N' = N - \sum k \). Then apply the standard formula \( \binom{N'+r-1}{r-1} \).