25*7=175 dollars
(1) 5*7=35,write down 5 and advance the 3
(2) 2*7=14, 14+3(from step one)=17 write down 7 and advance the 1
(3) since there is no more digits to do calculations, write down the 1
the answer is 175
(is this the mental math you re talking about?)
