Answer
(a) $249999$
(b) $4819$
(c) $3999951$
Work Step by Step
We use the special product formula:
(a) $501*499=(500+1)(500-1)=500^{2}-1=250000-1=249999$
(b) $79*61=(70+9)(70-9)=70^{2}-9^{2}=4900-81=4819$
(c) $2007*1993=(2000+7)(2000-7)=2000^{2}-7^{2}=4000000-49=3999951$
