Solve using cross-multiplication :

0.4x - 1.5y = 6.5

0.3x + 0.2y = 0.9

Four times a certain two digit number is seven times the number obtained on interchanging its digits. If the difference between the digits is 4; find the number.

The sum of a two digit number and the number obtained by interchanging the digits of the number is 121. If the digits of the number differ by 3, find the number.