Change 12 out of 40 to a percentage

Plz hurry evaluate the expression a+b where a=8 and b=19

In a two-digit number the units' digit is 7 more than the tens' digit. the number with digits reversed is three times as large as the sum of the original number and the two digits. find the number.

Cd is the perpendicular bisector of both xy and st, and cy=20. find xy.