Question 269458
Let 10t + u be the two digit number where t = tens digit and u = units digit.
From above we get
(i) {{{t + u = 12}}}
(ii) {{{10t + u + 15 = 6u}}}
step 1 - in (ii) subtract 6u from both sides and then subtract 15 from both sides to get
(iii) {{{10t - 5u = -15}}}
step 2 - multiply (i) by 5 to get
(iv) {{{5t + 5u = 60}}}
step 3 - add (iii) and (iv) to get
(v) {{{15t = 45}}}
step 4 - divide to get
t = 3
this means that u = 9.
The number we seek is
39.