Suppose that cross price elasticity for spinach and lettuce has been calculated to be +.4, what...

The cross price elasticity for spinach and lettuce is +0.4.

Since, the cross-price elasticity is positive, it shows that both spinach and lettuce are substitute goods. It shows that an increase in the price of spinach will cause an increase in the quantity demanded of lettuce and vice-versa.

If the price of spinach falls by 20%, the quantity demanded of lettuce will fall by:

{eq}\begin{align*} e &= \dfrac{{\% \Delta {Q_L}}}{{\% \Delta {P_S}}}\\ 0.4 &= - \dfrac{{\% \Delta {Q_L}}}{{20}}\\ \% \Delta {Q_L} &= - 8 \end{align*} {/eq}

So, the quantity demanded of lettuce will fall by 8%.