Multiplication and division by addition and subtraction I know that this sounds like the stupidest question possible, but I had to ask, is it possible to express any multiplication and division solely by addition and subtraction? Such as... 7*0.3 or 7/0.3?
You can express sine and cosine by a Taylor series so I wondered this purely out of curiosity. My searching didn't seem to come up with an answer anywhere though hence I'm asking.
 A: Yes it is possible 
$$x\cdot y=
\underbrace{x+x+\cdots+x  }_{\text{$y$ times}}$$
if $$\frac{x}{y}=n$$
$$0=x-\underbrace{(y+y+\cdots+y)}_{\text {$n$ times}}$$ 
$\frac{x}{y}$ means how many time we have to subtract $y$ from $x$ so that $x$ becomes 0
But here is one limitation $x,y$ belongs to Rational Numbers
A: Well,first let's look at the rules. For multiplication, 
if, $x,y\in \mathbb{Q}$
$$x.y=\underbrace{(x+x+x+x+...+x)}_\text{y times}$$
For division, if
$$ x/y=n$$
$$Then,x=y.n$$
$$means,x-\underbrace{(y+y+y+...+y)}_\text{n times}=0$$
Now,lets work on your given numbers.
Here, $$7*0.3=7*\dfrac{3}{10}$$
That means we have to multiply 7, 3 times.than yiels $\implies$
$$7*3=7+7+7=21$$
Now, we have $$\dfrac{21}{10}$$
Now,how many 10 do have a sum of 21,clearly it is more than 2 but not perfect 2.let's add 10, 2 times first.so we get,
$$21-(10+10)=1$$
1 is our remainder.now,let's think this 1 as 10.now we have to add another 10 to get this and now the remainder is 0. But as this was my imagination to think it as 10,i have to put the number behind a "decimal(point)" sign.so the answer is $2.1$
