The values of $t$ such that the matrix $\begin{bmatrix} 1 & 3 & 2 \\ 2 & 5 & t \\ 4 & 7-t & -6 \end{bmatrix}$ has no inverse,are

How many types of lichens are there based on their external morphology (growth form)?

Let $R_1$ be a relation defined by $R_1 = \{(a, b) | a \geq b, a, b \in \mathbb{R}\}$. Then $R_1$ is

$A$ nucleus $_Z{X^A}$ emits an $\alpha$-particle with velocity $v$. The recoil speed of the daughter nucleus is

$A$ cell of $emf$ $E$ and internal resistance $r$ is connected in series with an external resistance $nr$. Then,the ratio of the terminal potential difference to $emf$ is