The sum of two numbers is 18. twice the first number plus three times the second number equals 40. find the numbers.

a+b=18  

2a+3b=40  

Here's two solutions:

A) Multiply the first equation by (-2) & sum then the two equations, resulting ''b''. Or, multiply that first equation by (-3) Summing them you get ''a''. Whatever you obtain, introduce it into an equation and obtain the other one.  

B) You have a system of two equation with two unknown terms. Get one unknown term (e.g. ''a'') out of the first equation (a=18-b) & introduce it (18-b) into the other equation, the second one, instead of that unknown term, and you'll get this second equation written within just one unknown term ( 2(18-b)+3b=40 ; 36-2b+3b=40. With the term this way defined, you'll have also the first one.    

x+y=18; 2x+3y=40  

y=18-x, 2x+3(18-x)=40  

2x+54-3x=40  

-x=-14  

x=14  

14+y=18, y=4  

14+4=18  

2(14)+3(4)=40, 28+12=40

step-by-step explanation: i dont

the largest number of the four options is 1.8 x 10-2

you have it in scientific notation.

1.80 x 10-6   = 0.00000180

1.80 x 10-4   = 0.000180 (bigger, or "less small", than the former one)

so

1.8 x 10-2       = 0.0180. as you see. the "less small", or the biggest one of them all.

as you have negative exponent, it means you are dividing 1.8 by 100 (i.e. moving the dot towards left, two times).

if you had positive exponent for the scientific notation, power of tenths, you would be multiplying times 100 (moving the dot towards right) as below:

1.8 x (10)2 = 180

so. if you have negative exponent in scientific notation, move the dot toward left. if it is positive, toward right as many times the exponent tells you.