11. The temperature at noon is 75°F. The

temperature drops 3 degrees every half hour.

What is the temperature at 4 P. M.?

Assume temp T = T0 (75 0) at Noon (00), To find out T after 4 hours at 4 p. m. or at the 8th period, each period being 0.5 hour. If the relationship is linear, i. e if temperature is dropping linearly each half an hour by 3 0, the T is a staight line T=m*t + T0, in which the slope m=∇T/∇t = (-30/0.5 hr) = - 60/hour (negative slope decreasing)

So at 4 p. m. for t=4, temp T = 750 - 60/hour*4 hour = 750-240 = 510