Odd Factorial Sum Digit

The double factorial, denoted by $ n!\phantom{}!$, returns the product of all of the odd integers that are less than or equal to $ n $. For example, $ 7!\phantom{}!= 7 \times 5 \times 3 \times 1 $. What is the units digit of $ 1!\phantom{}!+ 3!\phantom{}!+ 5!\phantom{}!+ 7!\phantom{}!+ \cdots + 49!\phantom{}!$?

  • 1
  • 2
  • 3
  • +
  • 4
  • 5
  • 6
  • -
  • 7
  • 8
  • 9
  • $\frac{a}{b}$
  • .
  • 0
  • =
  • %
  • $a^n$
  • $a^{\circ}$
  • $a_n$
  • $\sqrt{}$
  • $\sqrt[n]{}$
  • $\pi$
  • $\ln{}$
  • $\log$
  • $\theta$
  • $\sin{}$
  • $\cos{}$
  • $\tan{}$
  • $($
  • $)$
  • $[$
  • $]$
  • $\cap$
  • $\cup$
  • $,$
  • $\infty$