(0.0042) (280,000)/4,900,000

(0.0042) (280,000)/4,900,000=

(4.2 x 10^-3) (2.8 x 10⁵)/ (4.9 x 10⁶) =

(4.2)(2.8)/(4.9) x 10^-3 x 10⁵ x 10^-6=

(4.2)(2.8)/(4.9) x 10^-4

2.4 x 10^-4

its 24 000 smeg head

(0.0042) (280,000)/4,900,000

$$\\=\frac{42*10^{-4}\;\;\times\;\;28*10^4}{49*10^5}\\\\
$divide top and bottom by 7$\\\\
=\frac{6*10^{-4}\;\;\times\;\;28*10^4}{7*10^5}\\\\
$divide top and bottom by 7 again$\\\\
=\frac{6*10^{-4}\;\;\times\;\;4*10^4}{10^5}\\\\
=\frac{6*4*10^{-4}*10^4}{10^5}\\\\
=\frac{24*10^{-4+4}}{10^5}\\\\
=\frac{24}{100000}\\\\
or\\\\
=24*10^{-5}\\\\
=2.4*10^1*10^{-5}\\\\
=2.4*10^{-4}$$

My answer is different from what you were expecting - I cannot spot a mistake bur maybe someone else can