Question 789137
Part "a" could make sense if you try to think about the units, and also try to simulate what is happening (mentally) as you read the description.  


At some time 0 days, fire covers 420 acres.  In day 1, fire covers {{{420+1*60}}} acres; in day 2, fire covers {{{420+2*60}}} acres;  and this pattern is easier to see than may be others are easy to see...  In day d, the fire covers {{{420+d*60}}} acres; but you would write this expression as {{{420+60d}}} acres.