a) ${ 6482+597+403+518}$
${= (6482+518)+(597+403)}$
${= 7000+1000=8000}$
b) ${590+160+210+410}$
${=(590+210)+(160+410)}$
${=800+570=1370}$
c) $\text{ 25 x 12 x 4 = (25 x 4) x 12 = 100 x 12=1200}$
d) $\text{615 x 101 = 615 x 100 + 615 = 61500 + 615 = 62115 }$
e) $\text{ 273 x 99 = 273 x 100 - 273 = 27300 - 273 = 27027 }$