How many numbers can be formed using digits 1 2 3 4 5 6 7 8 9 such that they are in increasing order(e.g. 12345=?iso-8859-1?Q?=2C_578?=)=?iso-8859-1?Q?=3F_=96_math.stackexchange...?=

How many numbers can be formed using digits 1 2 3 4 5 6 7 8 9 such that
they are in increasing order(e.g. 12345, 578)? – math.stackexchange...

Help me in how to solve this problem please. I will be grateful to you :)