investing math. How much did I invest?

Question

Sam invested a portion of 15,000 at 10% annual interest, and the rest at 6% annual interest. He earned $1260 how much was invested at 10%

Answer 1

a=the amount Sam invested at 10%
15000-a=the amount Sam invested at 6%
a*0,10+(15000-a)*0,06=1260
0,10*a+900-0,06a=1260
0,04*a=360
a=9000
9000 was invested at 10%

Answer 2

Let the investment portion be x.
It had a earning of 10% i.e. 0.1x

The balance investment left is (15,000-x)
It had a earning of 6% i.e. 0.06(15,000-x)

The total earning is 1,260

Equation set up to find x :

0.1x+0.06(15,000 - x) = 1,260

After setting up the equation, you should be able to solve by multiplication of the item with bracket first.
Then move in between left and right of the = sign so that
Addition become Subtraction
Subtraction become Addition
Multiplication become Division
Division become Multiplication and vice vise
to work out the following

0.1x+900-0.06x=1,260
0.1x-0.06x=1,260-900
0.04x=360
x=360/0.4
x=9,000

The portion of investment that earned a 10% annual interest is 9,000

Answer 3

Let x be amount invested at 10% APR
The following equation holds:
x * 10/100 + (15000 - x) *6/100 = 1260
Solving for x gives x = $$ 9,000

Answer 4

let a be the portion he first invested at 10%
the rest : 15,000 - a
we have
0.1*a + 0.06*(15,000 - a) = 1260
=> a = 9000