M, EEE, D. I, T, RR, AA, NN
\(\frac{R}{ }--\frac{E}{ }\)
Two empty places can be filled with identical letters [EE, AA, NN] \(\Rightarrow\) 3 ways
Two empty places, can be filled with distinct letters [M, E, D, I, T, R, A, N] \(\Rightarrow ^{8}P_{2}\)
\(\therefore\) Number of words 3+56=59
or
\(3+^{8}P_{2}=59\)
Foot of perpendicular from origin on a line passing through $(1, 1, 1)$ having direction ratios $\langle 2, 3, 4 \rangle$, is:
A line through $(1, 1, 1)$ and perpendicular to both $\hat{i} + 2\hat{j} + 2\hat{k}$ and $2\hat{i} + 2\hat{j} + \hat{k}$, let $(a, b, c)$ be foot of perpendicular from origin then $34 (a + b + c)$ is:
Object is placed at $40 \text{ cm}$ from spherical surface whose radius of curvature is $20 \text{ cm}$. Find height of image formed.

Permutation is the method or the act of arranging members of a set into an order or a sequence.
Combination is the method of forming subsets by selecting data from a larger set in a way that the selection order does not matter.