Part A
Total number of litres in the punch = grape juice + lemon lime soda
let grape juice be

, and lemon-lime soda be

the expression for the total number of litres in the punch is given by

We know the number of litres for the lemon-lime soda, hence the expression is

Part B:
40% of the punch is lemon-lime soda which is 2 litres
That means every 1 litre represents 20% of the punch
60% of the punch is grape-juice, hence 60÷20 = 3 litres of grape juice