You want to use the distributive property, i.e. the possibility to distribute a multiplication to both terms of an sum/subtraction.
In this case, if you think of 395 as of 400-5, you have
[tex] 5 \times 395 = 5 \times (400-5) [/tex]
And you can use the distributive property to write
[tex] 5 \times (400-5) = 5\times 400 - 5\times 5[/tex]
Both of these multiplications are easy to perform in your mind:
[tex] 5\times 400 - 5\times 5 = 2000 - 25 = 1975[/tex]
