The equivalent weight of an acid:

What is meant by $1 \ amu$ or $1 \ u$?

The equivalent weight of phosphoric acid $(H_3PO_4)$ in the reaction $NaOH + H_3PO_4 \to NaH_2PO_4 + H_2O$ is

$3.011 \times 10^{22}$ atoms of an element weigh $1.15 \ g$. The atomic mass of the element is: (in $amu$)

An element,$X$ has the following isotopic composition:
$^{200}X: 90\%$,$^{199}X: 8.0\%$,$^{202}X: 2.0\%$
The weighted average atomic mass of the naturally occurring element $X$ is closest to......$amu$.

Isotope	Relative abundance $(\%)$	Atomic mass $(u)$
$^{12}C$	$98.8$	$12$
$^{13}C$	$1.18$	$13.1$
$^{14}C$	$0.02$	$14.1$

From the above data,what is the average molecular mass of $CH_4$ containing all isotopes of carbon,assuming hydrogen is only $^{1}_{1}H$? (Given that atomic mass of hydrogen $= 1.008 \ u$)