How many nodal tissues (specialized cardiac musculature) are present in the human heart?

If the coefficient of $x$ in the expansion of $\left(x^2+\frac{k}{x}\right)^5$ is $270$,then $k$ is equal to

$A$ force applied by an engine on a train of mass $2.05 \times 10^6 \ kg$ changes its velocity from $5 \ m/s$ to $25 \ m/s$ in $5 \ minutes$. The power of the engine is (in $MW$)

$A$ circle with center at $(2,4)$ is such that the line $x+y+2=0$ cuts a chord of length $6$. The radius of the circle is

Consider the following reactions:
$3NiS + 2HNO_3 + 6HCl \rightarrow 3NiCl_2 + 2NO + 3S + 4H_2O$
$NiCl_2 + 2NH_4OH + 2(CH_3C(=NOH)C(=NOH)CH_3) \rightarrow B + 2NH_4Cl + 4H_2O$
The number of protons that do not involve in hydrogen bonding in the product $B$ is:

Let $A = \{1, 2, \{3, 4\}, 5\}$. Which of the following statements is incorrect and why?
$\varnothing \in A$