Simple

1*2+3-4:5=x

x =2+3-0.8

x=5-0.8

x=4.2

I did it like this:

1 x 2 + 3 - 4:5 = x

2 + 3 - 4:5 = x

5 - 4:5 = x

Since fractions can be written in ratios and vice versa, 5 - 4:5 can also mean:

5 - 4/5 = x

4.2 = x

So, x is 4.2

$1 × 2 + 3 - \dfrac{4}{5} = 2 + 3 - 0.8 = \color{#69047E}{\boxed{4.2}}$ .

$1 \times 2 + 3 - \frac{4}{5} = x$

$2 + 3 - 0.8 = x$

$x = \boxed{4.2}$