Formula for compound interest is
Interest = Amount - Principal
Interest = - P
Where r = rate in percent
t = time in years
For Jaina,
P = $300
r= 7%
t = 3 yrs
So, Interest = - 300
For Tomas
P = $400
r= 4%
t = 3 yrs
So, Interest = - 400
So pair of equations for Jaina and Tomas are
Interest = - 300
Interest = - 400
