Calvin invests $5000 at 5% p.a. Chloe invests $4000 at 7% p.a. How long will it take for their respective balances to be equal?
for interest "paid annually" (I'm guessing that's what p.a. means)
$\displaystyle A = P(1 + r)^t$
where A = account balance
P = initial principle
r = annual percentage rate
t = time in years
you want to know when the account balances are equal ...
$\displaystyle 5000(1.05)^t = 4000(1.07)^t$
solve for t