Greening up formula for multiple trades

[PioPio]

Hello,

What is the greening up formula if there are multiple trades? For instance, I have the following three sets of bets:
back bet back price lay bet lay price
1st trade £10.00 2 £10.00 1.8
2nd trade £5.00 2.1 £5.00 2
3rd trade £15.00 2.2 £15.00 2.1

after a while, the price goes down to 1.9(back) and 2.0(lay) and I want to green up for a profit.
What is the formula to be used to obtain the value of the bet?

Many thanks
Alberto