Question

is 6/5 equivalent to 80/100

Answer 1

$\frac{80}{100}$ 
the least common multiple of both is 20
$\frac{80/20}{100/20}$

$\frac{4}{5}$
No they are not equivalent.

Answer 2

You can tell right away that it's not, just by looking at them,
without doing any arithmetic.

-- 6/5 is more than ' 1 ', because the top is more than the bottom.

-- 80/100 is less than ' 1 ', because the top is less than the bottom.

So they can't be equivalent.