A kiosk on Brighton pier sells ice-creams. Daily sales may be modelled by the function

S(t,r)=5(t-6.5)(40-r)

where t denotes maximum daily temperature in C and r denotes daily rainfall in mm.

(a) How many ice-creams are sold on a day when it is 22C and there is 4 mm of rain?

(b) Use the small increments formula to estimate the change in sales due to a 1.5C rise in temperature and a 1 mm increase in rainfall from the levels in part (a)