Question

A 2 digit number is 5 times its units digit. the sum of the digits is 7. find the number

Answer 1

X - tens digit
y - units digit

$10x+y=5y\\ x+y=7\\\\ 10x=4y\\ x=7-y\\\\ 10(7-y)=4y\\ 70-10y=4y\\ 14y=70\\ y=5\\\\ x=7-5\\ x=2\\\\ 10\cdot2+5=\boxed{25}$