let x= the number that equal 7
let 7= the number that equal 77
7x+77y=350
x+11y=50
If y =1, x =39 ...... n would be 40
if y = 2, x =28 ....... n would be 30
if y = 3, x =17 ........n would be 20
etc..
So the only answer choice that fits is 40
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Source: Beat The GMAT — Problem Solving |
Using the equation:
7x+77y=350
X is the number of 7's and y is the number of 77's in the series. Now if we focus on the units digit of 350, how many 7's do we need to add, in order to come up with a zero in the units place of the answer? 10 sevens. Thus x + y = 10.
7+7+7+7+7+7+7+7+7+7 - adding them you get
7 X4 X1 X8 X5 X2 X9 X6 X3 X0
So you know that the answer will have to be a multiple of 10. The only answer choice that is a multiple of 10 is 40.
7x+77y=350
X is the number of 7's and y is the number of 77's in the series. Now if we focus on the units digit of 350, how many 7's do we need to add, in order to come up with a zero in the units place of the answer? 10 sevens. Thus x + y = 10.
7+7+7+7+7+7+7+7+7+7 - adding them you get
7 X4 X1 X8 X5 X2 X9 X6 X3 X0
So you know that the answer will have to be a multiple of 10. The only answer choice that is a multiple of 10 is 40.
