13.04-12.06 is the same as 1304-1206 except you need to replace the dec point two places from the right when you've finished the subtraction. To make the subtraction easier, do it in chunks: subtract 1204 leaving 100, then take away a further 2 leaving 98. Now put back the dec pt 2 places from the end: .98 and finally put zero in front to protect the little dec point: 0.98.