how do you calculate interest? ?

Depends on the compounding, assuming you're letting the interest stay in the account and not withdrawing it. If it's compounded monthly, for example, after the first month you'd have $60,000 x .035 divided by 12 (months). The interest is $175. So now you have $60,175. The second month's interest payment would be a little more, because now you're earning interest on $60,175. Rounding up, you have another $176. By the third month, you're earning interest on $60,351, etc. Most accounts compound monthly, but some compound daily, quarterly, or whatever. So with compounding you're making more than 3.5%. That's why you often see two interest rates provided- one is the simple interest rate, the other is the APY (Annual Percentage Yield) which reflects the compounding.

M = P( 1 + i )^n M is the final amount including the principal. P is the principal amount. i is the rate of interest per year. n is the number of years invested m = 700(1+0.06)^5 = £936.76