Rewrite 3/10 as a fractions that could be added easily to 21/100

Remember:
x/y times z/t=(xz)/(yt) and
x/y+z/y=(x+z)/y
conver the bottom number to the same number as the 21/100 or convert 10 to 100
obviously multiply by 10
but msut multiply by 1 to keep same value
 remember that 10/10=1 so
multiply 3/10 by 10/10
3/10 times 10/10=30/100
so add

30/100+21/100=(30+21)/100=51/100

Here is my answer, i'm sorry if you didn't understand my explanation