Lets solve this problem by a technique which we call unitary method.
So,
We have,
\(0.4 ~million ~accounts ~are ~added ~in~1~day. \\ \\ \frac{4}{10} ~million ~accounts ~are ~added ~in~1~day \\ \\ \frac{4}{10} * \frac{10}{4} ~million ~accounts ~are ~added ~in~1* \frac{10}{4}~day \\ \\ 1 ~million ~accounts ~are ~added ~in~ \frac{10}{4}~day \\ \\ 20 ~million ~accounts ~are ~added ~in~ \frac{10}{4}*20~day \\ \\ 20 ~million ~accounts ~are ~added ~in~ 50~days \\ \\\)
