A boy adds all natural numbers from 1 to 20, however he misses one number due to which the sum becomes 190. What is the number which the boy missed ?

$1+2+3+ldots .+n=frac{n(n+1)}{2}$

$herefore 1+2+3+ldots+20=frac{20 imes 21}{2}=210$

∴ Required number = 210 – 190 = 20