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