If half lives of a radioactive element, undergoing parallel path $\mathrm{\alpha }$-decay and $\mathrm{\beta }$-decay are 4 years and 12 years respectively, then percentage of element that remains after 12 years will be :

Number of half-life completed due to α decay is $12/4=3$
Number of half-life completed due to β decay is $12/12=1$
So total number of half-life completed is $3+1=4$
So the percentage of the element that remains after 12 years will be

$100/2^4=100/16=6.25\mathrm{\%}$