Question

2.1 x 1.5 Also what is 8.91 x1.26 ———

Answer 1

Assuming you want both of these done. I shall solve.

$21\cdot \:15 \ \textgreater \ \mathrm{Line\:up\:the\:numbers} \ \textgreater \ \begin{matrix}\space\space&2&1\\ \mathrm{x}&1&5\end{matrix}$

From there $\mathrm{Multiply\:without\:the\:decimal\:points,\:then\:put\:the\:decimal\:point\:in\:aswr}$
aswr = answer

$\mathrm{Multiply\:the\:top\:number\:by\:the\:bolded\:digit\:of\:the\:bottom\:number}$

$\begin{matrix}\space\space&\textbf{2}&\textbf{1}\\ \mathrm{x}&1&\textbf{5}\end{matrix} \ \textgreater \ \mathrm{Mutliply\:the\:bold\:numbers}: \:1\cdot \:5=5 \ \textgreater \ \frac{\begin{matrix}\space\space&2&\textbf{1}\\ \mathrm{x}&1&\textbf{5}\end{matrix}}{\begin{matrix}\space\space&\space\space&5\end{matrix}}$

$\mathrm{Mutliply\:the\:bold\:numbers}: \:2\cdot \:5=10$
$\mathrm{Carry\:}1\mathrm{\:to\:the\:column\:on\:the\:left\:and\:write\:}0\mathrm{\:in\:the\:result\:line}$

$\frac{\begin{matrix}\space\space&1&\space\space&\space\space\\ \space\space&\space\space&\textbf{2}&1\\ \mathrm{x}&\space\space&1&\textbf{5}\end{matrix}}{\begin{matrix}\space\space&\space\space&0&5\end{matrix}} \ \textgreater \ \mathrm{Add\:the\:carried\:digit,\:}1\mathrm{,\:to\:the\:result} \ \textgreater \ \frac{\begin{matrix}\space\space&1&\space\space&\space\space\\ \space\space&\space\space&2&1\\ \mathrm{x}&\space\space&1&5\end{matrix}}{\begin{matrix}\space\space&1&0&5\end{matrix}}$

$\mathrm{Multiply\:the\:top\:number\:by\:the\:bolded\:digit\:of\:the\:bottom\:number}$
$\frac{\begin{matrix}\space\space&\space\space&\textbf{2}&\textbf{1}\\ \space\space&\mathrm{x}&\textbf{1}&5\end{matrix}}{\begin{matrix}\space\space&1&0&5\end{matrix}}$

$\mathrm{Mutliply\:the\:bold\:numbers}: \:1\cdot \:1=1 \ \textgreater \ \frac{\begin{matrix}\space\space&\space\space&2&\textbf{1}\\ \space\space&\mathrm{x}&\textbf{1}&5\end{matrix}}{\begin{matrix}\space\space&1&0&5\\ \space\space&\space\space&1&\space\space\end{matrix}}$

$\mathrm{Mutliply\:the\:bold\:numbers}: \:2\cdot \:1=2 \ \textgreater \ \frac{\begin{matrix}\space\space&\space\space&\textbf{2}&1\\ \space\space&\mathrm{x}&\textbf{1}&5\end{matrix}}{\begin{matrix}\space\space&1&0&5\\ \space\space&2&1&\space\space\end{matrix}}$

$Add\:the\:rows\:to\:get\:the\:answer\;fill\;in\;zeros \ \textgreater \ \frac{\begin{matrix}\space\space&\space\space&2&1\\ \space\space&\mathrm{x}&1&5\end{matrix}}{\begin{matrix}\space\space&1&0&5\\ \space\space&2&1&0\end{matrix}}$

$\begin{matrix}\space\space&1&0&5\\ +&2&1&0\end{matrix} \ \textgreater \ \mathrm{Add\:the\:digits\:of\:the\:bolded\:column}: \:5+0=5$
$\frac{\begin{matrix}\space\space&1&0&\textbf{5}\\ +&2&1&\textbf{0}\end{matrix}}{\begin{matrix}\space\space&\space\space&\space\space&\textbf{5}\end{matrix}}$

$\mathrm{Add\:the\:digits\:of\:the\:bolded\:column}: \:0+1=1 \ \textgreater \ \frac{\begin{matrix}\space\space&1&\textbf{0}&5\\ +&2&\textbf{1}&0\end{matrix}}{\begin{matrix}\space\space&\space\space&\textbf{1}&5\end{matrix}}$

$\mathrm{Add\:the\:digits\:of\:the\:bolded\:column}: \:1+2=3 \ \textgreater \ \frac{\begin{matrix}\space\space&\textbf{1}&0&5\\ +&\textbf{2}&1&0\end{matrix}}{\begin{matrix}\space\space&\textbf{3}&1&5\end{matrix}} \ \textgreater \ 315$

$2.1\mathrm{\:has\:}1\mathrm{\:decimal\:place}, 1.5\mathrm{\:has\:}1\mathrm{\:decimal\:place}$
$\mathrm{Therefore,\:the\:answer\:has\:}2\mathrm{\:decimal\:places} \ \textgreater \ 3.15$

Hope this helps!

NOTE: Create another question for the second one as there is a character limit and then post the link on my profile page and from there I will assist you once more.