[justlurking]

You have two simultaneous equations:

The sum of two numbers is 65

X + Y = 65

And if the larger number is divided by the smaller number then the quotient is 3 and the remainder is 5

(3 * X) + 5 = Y

Now, substitute second equation into first:

X + (3 * X) + 5 = 65

Simplify it:

(4 * X) + 5 = 65

Subtract 5 from both sides:

(4 * X) = 60

And divide both sides by X:

X = 15

Now substitute the 15 back into the first equation:

15 + Y = 65

Subtract it 15 from both sides:

Y = 50

[justlurking]

Oops, a typo:

And divide both sides by X

should be:

And divide both sides by 4: