$($a$)$ Explain, using a two$-$period indifference curve framework $($with income arising in both periods, namely, present and future$),$ how an individual may respond to a decrease in the interest rate by decreasing or increasing savings. Please include references to the substitution effect, the income effect, present consumption and future consumption in your answer. $(50$ marks$)($b$)$ If leisure is a normal product for an individual, must that individual$$s labour supply curve be upward$-$sloping? Explain your answer. Explain why policy$-$makers might be interested in this issue? $(50$ marks$)$