Let A be the set of 4 digit numbers ${\mathrm{a}}_1{\mathrm{a}}_2{\mathrm{a}}_3{\mathrm{a}}_4$ where ${\mathrm{a}}_1>{\mathrm{a}}_2>{\mathrm{a}}_3>{\mathrm{a}}_4$ then n (A) is equals to 
 

Let A be the set of 4 digit numbers ${\mathrm{a}}_1{\mathrm{a}}_2{\mathrm{a}}_3{\mathrm{a}}_4$ where ${\mathrm{a}}_1>{\mathrm{a}}_2>{\mathrm{a}}_3>{\mathrm{a}}_4$ then n (A) is equals to 
 

number of ways of selecting 4 digits from 1 to 10=${ }^{10}{C}_4=210$