My calculation is getting converted to integer by default(KSH)

I am assigning a decimal value to a variable which is an output from an oracle command. when I echo the variable I see the decimal value. For example, echo $a is showing me .7214 but when I do calculations on this, it is defaulted to integer. For example, b=$((a+(a*20/100))) is rounding to 1. How do I avoid rounding this to integer.